Renal function of eighteen children with solitary kidney (14 congenital and 4 acquired) was assessed using DMSA renal uptake rate and urinary excretion of alpha 1-MG, beta 2-MG, NAG and microalbumin. Seven of the cases were associated with vesicoureteral reflux (VUR) and two with congenital hydronephrosis. These anomalies have been already treated surgically before entering this study. DMSA renal uptake rate of 8 children was the same as that of the controls (51.8 +/- 3.7%; mean +/- SD). However, the uptake rate of 10 cases were more than 2SD below the mean of the controls. In five of them, including two children with no other urinary tract anomaly, the uptake rate was less than 70% of the mean of the controls. Half of 14 children evaluated with alpha 1-MG showed high values. In 6 of these 7 cases, DMSA renal uptake rate was more than 2SD below the mean of the controls. Urinary microalbumin was not correlated with urinary alpha 1-MG or DMSA renal uptake rate. Three of 4 children with high values of urinary microalbumin were more than 10 years old. Nine of 12 children evaluated with all the indices mentioned above showed high values of urinary alpha 1-MG and/or microalbumin. Low DMSA renal uptake rate was revealed in 7 out of these 9 children including two cases without urinary tract anomaly. These results indicate that, in the cases with solitary kidney even though no other urinary tract anomaly is recognized, renal overload may have already developed in early life.