We studied the sonographic findings and the changes in renal function seen on long-term follow-up of children who had the initial diagnosis of autosomal recessive polycystic kidney disease made in the neonatal period. The case records and sonograms of 14 children with biopsy evidence of autosomal recessive polycystic kidney disease were evaluated. Nine children who survived the neonatal period were followed up for a mean of 13 years (range, 5-19 years) after diagnosis and form the basis of this study. Serial changes in renal size, echogenicity, and function were assessed sonographically. The imaging findings were compared with those described in published reports. The sonographic findings showed that five of the nine children had a decrease in renal size, and three had stable renal size over a minimum follow-up period of 5 years. Only one of the nine survivors showed progressive increase in renal size. All had increased cortical echogenicity and large kidneys. Three patients showed a subjective change in renal echogenicity over time. A change in the echogenic pattern to one that resembles autosomal dominant polycystic kidney disease was noted with no evidence of increase in size of the kidneys. None of the surviving children had renal stones or massively enlarged kidneys. The renal function of seven of the nine survivors has remained stable with creatinine clearance nearly normal (> 60 ml/min/1.73 m2), and there was no correlation between renal size and renal function. In patients with autosomal recessive polycystic kidney disease who survive the neonatal period, kidney size as seen on sonograms does not continue to increase despite the patients' linear growth and maintained normal renal function. Rather, a decrease in kidney size and change in echogenicity occurs, producing a pattern that is similar to that seen on sonograms of patients with autosomal dominant polycystic kidney disease but without the marked increase in kidney size that occurs in that entity. This changing cystic pattern on follow-up sonograms may be the reason that previous descriptions of the sonographic findings in cases of autosomal recessive polycystic kidney disease have varied and why a decrease in size may not herald deteriorating renal function.