Tests for immunoglobulin M (IgM) antibody to group B coxsackieviruses were performed on sera from 259 patients with a clinical diagnosis of pericarditis, myocarditis, or pleurodynia on whom there were no definitive serological or virus isolation findings to establish a viral etiology, and on 259 "control" patients with clinical diagnoses of viral or mycoplasmal pneumonia or pneumonitis. IgM antibodies to coxsackievirus types B1, B3, B4, B5, and B6 were detected by a micro-immunodiffusion technique, and antibodies to virus type B2 were detected by reduction of neutralizing antibodies with ethanethiol. Of the patients with pericarditis, myocarditis, or pleurodynia, 27% (70) had IgM antibody to group B coxsackieviruses, as compared with 8% in the control group. On retrospective review of the clinical diagnosis, some of the patients in the control group with IgM antibody were found to have had additional clinical findings which could be attributed to a coxsackievirus infection. Coxsackievirus IgM antibody was demonstrable in 30% of 113 patients in the study group for whom virus isolation had been attempted with negative results. The presence of coxsackievirus IgM is discussed in relation to the time of serum collection, age of the patients, and month of onset of illness.