Serial determinations of the numbers of peripheral blood T-lymphocyte subpopulations (T-helper/inducer and T-suppressor/cytotoxic cells) were made using monoclonal antibodies in 12 patients with haemophilia who were HIV antibody positive. Eleven patients were clinically severe and one clinically moderate. Follow-up studies for 12-42 months in nine patients showed persisting abnormalities in five patients, abnormalities which developed in three patients who were initially normal, and a return to normal findings in one patient who was initially abnormal. The remaining three patients were followed up for 3-4 months; two showed persisting abnormalities and one a return to normal. Persistent generalized lymphadenopathy was found in one patient who showed raised levels of T-suppressor cells initially and low levels of T-helper cells in the follow-up, but low T-helper cells persisting for up to 12 months were not necessarily associated with disease.