The occurrence of hospital-induced complications on a university medical service was documented in the prospective investigation of over a thousand patients. The reported episodes were the untoward consequences of acceptable medical care in diagnosis and therapy. During the 8-month study, 240 episodes occurred in 198 patients. In 105 patients, hospitalization was either prolonged by an adverse episode or the manifestations were not yet resolved at time of discharge. Thus, 20% of the patients admitted to the medical wards experienced one or more untoward episodes and 10% had a prolonged or unresolved episode. The severity of the 240 episodes was minor in 110, moderate in 82, and major in 48, of which 16 ended fatally. Patients encountering noxious episodes had a mean total hospitalization of 28.7 days, compared with 11.4 days in other patients. The risk of having such episodes seemed directly related to the length of time spent in the hospital. The number and variety of these reactions emphasizes the magnitude and scope of hazards to which the hospitalized patient is exposed. A judicious selection of diagnostic and therapeutic measures can be made only with knowledge of these potential hazards as well as the proposed benefits.