The purpose of this study was to determine the prevalence, clinical characteristics, prenatal diagnosis and occurrence of other birth defects with abdominal wall defects in births and terminations of pregnancy in South Australia (SA) and Western Australia (WA) over the period 1980-90. Cases of gastroschisis, exomphalos, bladder exstrophy, cloacal exstrophy and body stalk anomaly were ascertained from the WA Birth Defects Registry (1980-90) and the SA Birth Defects Register (1986-90). The registers are comparable population-based data collections with information on livebirths and stillbirths of at least 400 g birthweight or 20 weeks' gestation, and terminations of pregnancy for fetal abnormality. The prevalence of gastroschisis was 1.65/10,000 births (59 cases) and of exomphalos 2.90/10,000 births (104 cases). There was no significant difference in prevalence of exomphalos or gastroschisis between SA and WA for the years 1986-90. However, if data from WA for the years 1980-85 were included, SA had a significantly higher prevalence of exomphalos (prevalence ratio 1.71, confidence interval [CI] 1.16-2.55), although not of gastroschisis (prevalence ratio 1.35, CI 0.79-2.32). Exomphalos was significantly more common in mothers < 20 years (odds ratio [OR] 2.45, CI 1.22-4.86) and in mothers of 40 years or older (OR 5.65, CI 1.69-16.77). Gastroschisis was more common in younger mothers (OR 8.76, CI 4.02-19.32). Both exomphalos and gastroschisis were associated with low birthweight, prematurity, intrauterine growth retardation and caesarean section. The reason for the higher prevalence of exomphalos in SA than WA was not clear, but may be related to differences in prenatal diagnosis. The association between maternal age < 20 years and exomphalos raises the possibility of common factors in the aetiology of gastroschisis and exomphalos.