Immunoblot analysis using mouse monoclonal antibody to human IgM detected IgM to cytomegalovirus (CMV)-specific proteins (150, 42, 38, 32, and 28 kDa) in 74 (38%) of 197 seropositive serum samples from 197 individuals in three subject groups: 43 surgical patients, 31 patients with solid tumors, and 123 healthy individuals. Logistic regression analysis revealed that the presence of IgM to CMV proteins was not associated with a specific subject group but was associated with age (P less than .0001), gender (P less than .005), and IgG titer (P less than .03). The rate of IgM positivity increased from 15% (8 of 54) for those less than 20 y to 63% (37 of 59) for those greater than 60 y. Twenty-three (29%) of 80 males were IgM-positive compared with 49 (44%) of 111 females. Immunoglobulin G titers to CMV were significantly associated with age, gender, and IgM positivity. These data suggest that CMV reactivation in immunocompetent seropositive individuals may occur frequently and increase with age.