The course and outcome of a disease process is dependent upon factors such as the disease-causing agent and its cell tropism, defense mechanisms of the host, genetic resistance of the species or breed affected, as well as the age, nutritional status and hormonal levels of the affected animal. When haematology, clinical biochemistry and cytology test results are combined with other laboratory procedures, complete physical examination and also with the history of the patient, a veterinarian is well armed to arrive at a definitive diagnosis, make a certain prognosis (good, poor or guarded) and can also make a concluding statement on the efficacy of the instituted therapy. In clinical biochemistry, demonstration of specific enzyme activity and concentration of analytes in serum/plasma facilitates the disease diagnosis. Also, evaluation of haematology, clinical biochemistry and diagnostic cytology tests can help establish the presence or absence of diseases of internal organs, and by serial performance of these tests, may help to determine whether a disease process remains static, progressive or regressive. This review therefore provides the haematological, serum biochemical and cytological characteristics of diseases caused by the main bacteria, viruses, fungi, protozoa, helminths, arthropods, nutritional deficiencies, endocrine disturbances, neoplasm, allergy, toxins (phytoxins and zootoxins) and inorganic poisons in horses.