Homozygous cystinuria was diagnosed in 45 children and 19 of their siblings in the course of routine urine screening of 6-week-old infants in New South Wales. These children were followed for up to 14 years. During this time there were 5 clinical episodes of renal disease which could be ascribed to cystinuria. There was normal mental development in all the children except one. Of 49 children over 3 years, 4 had height centiles less than the midparent height centile, while 45 had height centiles equal to or above the midparent centiles. Family testing in these 45 cases showed that 60% were type I cystinurics, and 35% were of the mixed or compound type (5% were not classified). Data from the parents and grandparents showed that renal tract calculi had occurred in 14 of them. This study shows that children with homozygous cystinuria, detected by urinary screening in infancy, rarely have renal symptoms. Mental development was normal as was growth in height. There was an increased incidence of noncystine stone formation among the relatives of these children. The incidence of homozygous cystinuria in New South Wales in one in 17 286.