We have studied the effect of the HIV epidemic on the incidence of tuberculosis in an inner city area of London which has a high incidence of acquired immunodeficiency syndrome (AIDS). During the period 1985-1991, there was a steady increase in the number of new AIDS cases, whilst the numbers of notifications and laboratory isolates of Mycobacterium tuberculosis remained largely unchanged. Before 1990 there were few cases of tuberculosis in HIV-infected individuals but in the past 2 years there has been a marked increase. In 44% of patients, the site of infection was pulmonary, in 39% extrapulmonary and in 17% pulmonary and extrapulmonary. There were no previous manifestations of immunodeficiency in 58% of patients. This is the first study to show an association between tuberculosis and HIV in the U.K. and supports the suggestion that there is an increased incidence of tuberculosis in patients with HIV infection who are not intravenous drug users.