Using a 1982-1983 regional network data bank of 33,545 delivered infants, a study was conducted comparing 574 macrosomic infants weighing greater than 4500 g to a control group of 18,739 infants whose birth weights were 2500-3499 g. Macrosomic infants occurred in 1.7% of the deliveries. Women delivering macrosomic infants were significantly older, of higher parity, more obese (greater than 90 kg), and more frequently diabetic and postmature (longer than 42 wk) than the controls. The women having macrosomic infants had a higher frequency of cesarean deliveries. The macrosomic infants were more often male and had more birth trauma and shoulder dystocia, higher death rates and lower Apgar scores. Five min Apgar scores were lowest in the very macrosomic subgroup (greater than 5000 g). The high-risk group triad included obesity, diabetes and postdates and had a macrosomia frequency of 5-14%. Macrosomic infants delivered by cesarean section had significantly fewer birth injuries. Because of these serious perinatal problems, women at risk should be screened for macrosomic infants; if found, they should be delivered electively by cesarean section.