Tissue sections from 119 cats that died or were euthanatized (1952-1990) because of toxoplasmosis-like illness were reexamined for Toxoplasma gondii by direct microscopy and immunohistochemical staining with anti-T gondii serum. Clinical and pathologic data from 100 of these cats with histologically verified toxoplasmosis were then analyzed. Of these 100 cats, 36 were considered to have generalized toxoplasmosis, 26 predominantly pulmonary lesions, 16 abdominal, 2 hepatic, 1 pancreatic, 1 cardiac, 2 cutaneous, 7 neurologic, and 9 had neonatal toxoplasmosis. In 14 cats, concurrent microbial infections or other maladies were seen. Cats were 2 weeks to 16 years old (median, 2 years; mean, 4 years). Sixty-five cats were males and 34 were females; sex was not recorded for 1 cat. Of 67 cats that had rectal temperatures recorded, 49 (73%) had fever (40.0 to 41.7 C). Dyspnea, polypnea, and signs of abdominal discomfort were frequently observed. Toxoplasmosis had been confirmed antemortem in 8 cats; 4 had a serum antibody titer to T gondii of ≥ 1:1,024; and T gondii had been found in cytologic evaluation of tracheal aspirates from 2 cats and pleural fluid from 1 cat, as well as in a biopsy specimen of a mesenteric lymph node from another. Of the 15 cats with T gondii serum-antibody titers determined by the Sabin-Feldman dye test, 6 had no antibody detected in 1:4 dilution of their serum. Indirect fluorescent antibody titers were found in 10 of 10 cats' sera tested. Forty-one eyes from 27 of the cats were examined microscopically. Twenty-two of the 27 cats (81.5%) had evidence of intraocular inflammation in one or both eyes. Multifocal iridocyclochoroiditis was the most common lesion and was seen in 18 (81.8%) of the cats with ophthalmitis. The ciliary body was the most often severely affected portion of the uvea. Of the 22 cats with ocular toxoplasmosis, T gondii was found in eyes of 10. Toxoplasma gondii was found in the retina of 5 cats, the choroid of 2, the optic nerve of 1, the iris of 3, and the ciliary body of 4. Toxoplasma gondii was identified in 80% of 55 brains, 70.0% of 90 livers, 76.7% of 86 lungs, 64.4% of 45 pancreata, 62.7% of 59 hearts, 45.8% of 72 spleens, 41.5% of 65 intestines, 17.7% of 61 kidneys, and 60.0% of 30 adrenal glands. Tissue sections from 119 cats that died or were euthanatized (1952-1990) because of toxoplasmosis-like illness were reexamined for Toxoplasma gondii by direct microscopy and immunohistochemical staining with anti-T gondii serum. Clinical and pathologic data from 100 of these cats with histologically verified toxoplasmosis were then analyzed. Of these 100 cats, 36 were considered to have generalized toxoplasmosis, 26 predominantly pulmonary lesions, 16 abdominal, 2 hepatic, 1 pancreatic, 1 cardiac, 2 cutaneous, 7 neurologic, and 9 had neonatal toxoplasmosis. In 14 cats, concurrent microbial infections or other maladies were seen. Cats were 2 weeks to 16 years old (median, 2 years; mean, 4 years). Sixty-five cats were males and 34 were females; sex was not recorded for 1 cat. Of 67 cats that had rectal temperatures recorded, 49 (73%) had fever (40.0 to 41.7 C). Dyspnea, polypnea, and signs of abdominal discomfort were frequently observed. Toxoplasmosis had been confirmed antemortem in 8 cats; 4 had a serum antibody titer to T gondii of ≥ 1:1,024; and T gondii had been found in cytologic evaluation of tracheal aspirates from 2 cats and pleural fluid from 1 cat, as well as in a biopsy specimen of a mesenteric lymph node from another. Of the 15 cats with T gondii serum-antibody titers determined by the Sabin-Feldman dye test, 6 had no antibody detected in 1:4 dilution of their serum. Indirect fluorescent antibody titers were found in 10 of 10 cats' sera tested. Forty-one eyes from 27 of the cats were examined microscopically. Twenty-two of the 27 cats (81.5%) had evidence of intraocular inflammation in one or both eyes. Multifocal iridocyclochoroiditis was the most common lesion and was seen in 18 (81.8%) of the cats with ophthalmitis. The ciliary body was the most often severely affected portion of the uvea. Of the 22 cats with ocular toxoplasmosis, T gondii was found in eyes of 10. Toxoplasma gondii was found in the retina of 5 cats, the choroid of 2, the optic nerve of 1, the iris of 3, and the ciliary body of 4. Toxoplasma gondii was identified in 80% of 55 brains, 70.0% of 90 livers, 76.7% of 86 lungs, 64.4% of 45 pancreata, 62.7% of 59 hearts, 45.8% of 72 spleens, 41.5% of 65 intestines, 17.7% of 61 kidneys, and 60.0% of 30 adrenal glands.