To detect cytomegalovirus (CMV) infections, a total of 1,074 cultures of urine, saliva, or blood were collected weekly from 43 consecutive patients undergoing allogeneic bone marrow transplantation. Twenty-three patients were seronegative before transplant and primary infection occurred in 2 (9%). Twenty patients were initially seropositive and recurrent infections occurred in 5 (25%). Three patients in the recurrent group had proven CMV pneumonitis; viraemia was detected in two recipients, while the third had CMV isolated only from bronchial lavage fluid. The serological response of the 43 patients was defined by testing 559 serial sera for specific IgG and IgM antibodies by radioimmunoassay. Passive acquisition of IgG antibodies from blood products was found in 78% of initially seronegative recipients. One patient with primary infection responded in a pattern typical of immunocompetent individuals with long-term production of specific IgG and transient production of specific IgM antibodies. The second patient also had a typical response, but this was delayed until several weeks after the start of virus excretion. In patients with recurrent infections, specific IgM production did not correlate with episodes of virus excretion. Three of five such patients failed to mount a specific IgM response, and these were the only patients in the study to develop CMV pneumonitis. We conclude that CMV infection in bone marrow recipients can only be diagnosed by detection of virus; therefore, the ability of these patients to mount humoral immune responses should not be relied upon for diagnostic purposes.