The aim of the study was to assess serum cystatin C level in children with a congenital solitary kidney, depending on their age and compensatory overgrowth of the kidney. The study group (I) consisted of 36 children, 3-21 years of age (median 10.8 years), with a congenital solitary kidney and no other urinary defects. The control group (C) contained 36 healthy children, 5-21 years old (median 10.9 years). Nephelometric methods were used to determine serum cystatin C level, the Jaffe method to assess creatinine concentration and the Schwartz formula to estimate glomerular filtration rate. Kidney length was measured with the patient in a supine position, and overgrowth was estimated (O%) in comparison with the respective kidney in the control group. Serum cystatin C level in group I was higher than that in the control group (P<0.05). Increased values, above 0.95 mg/l, were found in 16/36 (44%) children aged 12-21 years. Glomerular filtration rate (GFR, estimated by the Schwartz formula) and creatinine level in group I were similar to those of the control group (P>0.05). Increased kidney length was found (median 18.2%). Cystatin C concentration was positively correlated with O% (r=0.406, P<0.01) and kidney length to child height ratio (L/H) (r=0.376, P<0.05). We conclude that Increased serum cystatin C concentration in patients with a unilateral congenital solitary kidney occurs after 12 years of age and correlates with compensatory overgrowth of the kidney.