Drug reactions are common in HIV-1 disease, with the incidence having been reported to increase with increasing stage and with CD4+ T-cell counts below 200/microliters. However, there have been numerous reports of patients in which rechallenge, dosing changes or continued therapy have resulted in no recurrence or else clearing of the eruption. We followed 974 HIV-1-positive patients for 46 months as a part of a military study of HIV-1 disease. Within this group there were a total of 283 drug eruptions, with cutaneous manifestations in 201 patients in which clinical characteristics were noted and 86 patients in which cutaneous biopsies were performed. Serological evidence of reactivation or acute Epstein-Barr virus (EBV) or cytomegalovirus (CMV) infections were also noted, as well as peripheral eosinophilia. The incidence of drug eruptions significantly increased with increasing Walter Reed stage and decreasing CD4 counts and CD4/CD8 ratio, as well as with increasing age and in patients with increased numbers of other dermatological diagnoses. In addition, white patients had significantly more drug eruptions than did black. Serological or culture evidence of acute or reactivated EBV or CMV was significantly increased in patients with drug eruptions. The majority of the eruptions were maculopapular or morbilliform with a predominantly perivascular mononuclear cell infiltrate. HIV-1 positive patients have an increased incidence of drug reactions, the incidence having been reported to increase in patients with less than 200 CD4+ T cells/microliter. However, at very low T4 counts, especially those less than 25/microliter, and at a CD4/CD8 ratio of less than 0.10, the probability of reactions to trimethoprim-sulphamethoxazole (TMP-SMZ) is decreased in late-stage HIV-1 patients. Maculopapular or morbilliform eruptions are the most common clinical presentations, often accompanied by one or more of the following: fever, arthralgias, eosinophilia, and serum transaminase elevation. Histologically the majority of these eruptions show a perivascular mononuclear cell infiltrate, sometimes with focal interface changes and apoptotic, necrotic cells within the epidermis. Acute hypersensitivity reactions and toxic epidermal necrolysis (TEN) or Stevens-Johnson's syndrome (SJS) with diffuse epidermal apoptosis and necrosis have also been less commonly described. In a study of cutaneous manifestations in an HIV-1 positive military population, drug reactions were evaluated in terms of clinical features, histopathology, demographic features and laboratory findings.