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A 46-year-old male patient was referred from a peripheral hospital with a 5 days history of high fever, dyspnea and respiratory deterioration. Direct immunofluorescence examination of bronchoalveolar fluid repeatedly showed clusters of Pneumocystis carinii. High-dose sulfamethoxazole-trimethoprim therapy was initiated and the patient recovered promptly during the following days. This otherwise healthy patient's past history was unremarkable in terms of prior infectious diseases. There was no evidence of immunodeficiency and he was not taking medication. Antibodies against HIV-1 were repeatedly negative, as were the assay for p24-antigen, PCR for HIV-DNA and HIV culture. Subpopulations of lymphocytes showed normal values. Analysis of the IgG fractions revealed a decreased subclass 2 fraction. Functional assays showed decreased biological binding capacity of this subclass 2 IgG to polysaccharide antigens. A four-fold increase of cytomegalovirus (CMV) IgG titer suggested a concomitant CMV infection or reactivation. As CMV infection is known to cause transient cellular immunodeficiency, reactivated CMV infection, in concert with IgG subclass 2 deficiency, could be a predisposing factor for P. carinii infection in this patient.