A marked increase or "epidemic" in ventricular septal defects (VSD) in recent years has been reported by the Center for Disease Control. Many pediatric cardiologists believe that this increase is simply a reflection of more intensive diagnosis and evaluation of infants throughout the country. Yet to our knowledge there has been no objective evidence for this explanation. We evaluated this possibility by considering records on live births occurring in 1970-1983 in the counties surrounding Albany, New York. In that period a single group of pediatric cardiologists has been evaluating all infants with suspected or confirmed cardiac defects in this area. We limited this analysis to ventricular septal defects unassociated with any cardiac syndrome complex. Thus, VSDs occurring as part of cyanotic heart disease or other complex cardiac "syndromes" were excluded. Consistent with the reported national trend, the estimated prevalence rate of ventricular septal defects diagnosed under 1 year of age in this period has increased from 1.0 per 1,000 live births in 1970 to 4.0 per 1,000 in 1983.(ABSTRACT TRUNCATED AT 250 WORDS)