Objectives: although Mycobacterium kansasii infection has long been endemic in the U.K., the disease burden and characteristics of infection in the HIV-seropositive population has not been well documented. This study addresses these issues in an inner city population that comprises a quarter of all cases of acquired immunodeficiency syndrome (AIDS) reported in the U.K. Methods: retrospective review of case notes from two inner London specialist HIV Units. Results: twenty-nine cases of M. kansasii infection were identified, with case notes available for review in 26. Ten had pulmonary disease and nine had disseminated infection: a further seven patients appeared simply to be colonized (two respiratory and five gastrointestinal); M. kansasii was isolated from stool in over a third (nine of 26) of cases. Disseminated M. kansasii infection occurred in 0.44% of AIDS cases seen in our two units and all isolates were resistant to isoniazid in vitro. A clinical response achieved in 11 of the 13 patients with M. kansasii-related disease who received anti-mycobacterial therapy. All four patients who relapsed following initial clinical response to therapy had received sub-optimal treatment. Conclusions: the incidence of disseminated M. kansasii infection in HIV-infected individuals in the U.K. is similar to that seen in those from high prevalence regions of the U.S.A., and anti-mycobacterial therapy leads to a clinical response in the majority of patients with HIV and M. kansasii co-infection. The frequent isolation of M. kansasii from the stool suggests that the gastrointestinal tract may be a significant source of disseminated infection.