Over a 5-year duration a total number of 57 881 tissue specimens from hospitalized patients were evaluated clinicopathologically for evidence of adverse tissue reactions to drugs. The incidence of alleged adverse tissue reactions was 3.7% of which 1.5% were definitely shown to be caused by a specific pharmacological agent. Regardless of the category of the adverse tissue reaction, the frequency of drug-related responses occurred most often in females, especially between the ages of 41 and 60 years. The target tissue affected adversely most often was endometrium where hyperplasia was the predominant response noted. An oestrogenic preparation was found most frequently to be implicated in the observed adverse tissue reaction. Evaluation of adverse drug reactions with the use of morphologic technique provides a rigorous and reliable mechanism to monitor deleterious effects in hospitalized patients.