40 forensic SIDS cases were found to have a significantly higher rate of serologic evidence of recent influenza A (H 3 N 2) infection than did matched controls. In contrast, the SIDS cases had serologically no significantly increased infection rate with influenza H 1 N 1 and B virus, parainfluenza virus, RS virus, adenovirus, and cytomegalovirus. SIDS cases with recent influenza infection had a significantly higher rate of histological findings as described for primary viral pneumonia than did SIDS cases without influenza infection. SIDS cases with recent influenza infection occurred much more frequently during epidemic than during interepidemic influenza A (H 3 N 2) periods. Our results confirm previous reports that SIDS cases have an increased rate of respiratory virus infections. However, they cannot prove a causal relationship between influenza infection and death. Since our SIDS cases comprised 75 per cent of cases aged more than three months, our results pertain essentially to cases of this age group.