While continuing a study of immunologic and virologic aspects of the postpericardiotomy syndrome (PPS) in infants and children, we undertook a companion study in adults. Whereas the incidence in 119 children was 27.8%, as is usual for our hospital, the incidence in 142 adults was 17.6%. Most of the operations (95 of 142) were performed on patients who were 50-69 years old. Incidence decreased as age increased: 28.5% for ages less than 40 years, 19.7% for ages 40--59 years, 14.2% for ages 60--69 years, and 10% for ages 70 years and older. Heart-reactive antibody in high titer was found in all patients with PPS; a rise in antiviral antibody to one or more of a battery of eight common viruses (adenovirus, cytomegalovirus, and coxsackie-virus B 1 to 6) was identified in 68% of these with PPS but in only 5% of those without clinical manifestations. We conclude that in adults, as in children, PPS may be an autoimmune phenomenon triggered by viral illness.