A serological study was carried out in Ibadan, Nigeria to determine the prevalence of inapparent infection with cytomegalovirus among tuberculosis patients and its relation to the epidemiology of the virus in this environment. A total of 360 sera from 161 tuberculosis patients, 89 patients other than tuberculosis and 110 healthy voluntary blood donors were examined for antibody to cytomegalovirus by the complement fixation test. One hundred and forty-one (87.6%) of the tuberculosis patients, 45 (50.6%) patients other than tuberculosis and 60 (54.6%) healthy blood donors had a complement fixing antibody to cytomegalovirus. Although the total prevalence (68.3%) of complement fixing antibody against cytomegalovirus in all the groups is high, it is higher among the tuberculosis patients than the other groups in the same environment. This shows that tuberculosis which is still a major health problem in Nigeria may be playing a significant role in the spread of cytomegalovirus in the country.