The immunocompromise syndrome in homosexual men in the USA is thought to be associated with cytomegalovirus (CMV) infection and nitrite intake. Such men often have a lymphopenia. In a clinic in London 76% of 46 unselected homosexual men and 50% of 76 heterosexual controls had serum CMV IgG antibody at a titre of 1/16 or more (p less than 0.01). No case of excretion of CMV in the urine was found. Thirty per cent of the homosexual men admitted to using nitrites. These figures suggest that this population of homosexual men uses nitrites less often than their counterparts in the USA and is less likely to have evidence of active or past CMV infection. In addition, these male homosexual patients seem to be less promiscuous than those reported from the USA. Mean accurate total and differential white blood cell counts, using the Haemalog D automatic white cell counter, were no different in homosexual men (and various at risk subgroups of them) than heterosexuals, suggesting that the prodrome to the immunocompromise syndrome was not present in the London clinic population.