The titers of IgG and IgA to Pneumocystis carinii in 36 AIDS patients did not differ significantly from those in 31 controls. Only 2/15 patients (13%) with P. carinii pneumonia (PCP) had titers of IgM antibodies greater than or equal to 5, which is significantly less frequent than in 32 controls (62%) and in 21 AIDS patients without PCP (43%). The risk of PCP was 5 times higher in patients without IgM antibodies to P. carinii than in patients who had these antibodies. A significantly higher percentage of those without PCP (57%) showed increasing titers of IgM antibodies to P. carinii in the second of paired samples taken about 6 months apart, compared with whose with PCP (9%; p = 0.05). All patients had high titers of antibodies to CMV and HSV and normal total concentrations of immunoglobulins. None of the patients responded in lymphocyte transformation to P. carinii, CMV, or HSV antigens. There is no obvious explanation to the selective lack of IgM antibodies to P. carinii in patients with PCP. Lack of IgM antibodies may be a marker for an immunodeficiency to P. carinii.