Cystinuria is an autosomal recessive disorder characterized with abnormal 
tubular reabsorption of cystine and dibasic amino acids leading to cystine 
urolithiasis. The classical form is caused by mutations in the SLC3A1 gene (OMIM 
220100). The cornerstone of the treatment is high hydration and alkalization of 
the urine to achieve urine pH between 7.0 and 7.5, at which point, cystine 
solubility in the urine is optimal. These measures very often fail, and thus 
addition of sulfhydryl agents like penicillamine and tiopronin 
(mercaptopropionyl glycine) is recommended. Herein, we report a 3-year-old boy 
with cystinuria resulting in recurrent nephrolithiasis requiring surgery and 
extracorporeal shock wave lithotripsy. Nine months after introduction of 
tiopronin, the boy manifested generalized edema, oliguria, and biochemical 
indices of nephrotic syndrome. Tiopronin was withdrawn, and the boy was given 
only supportive treatment. Within 10 days, he entered into clinical and 
biochemical remission. Pediatricians should be aware of this adverse effect of 
tiopronin, and therefore, testing of the urine with strips or sulfosalicylic 
acid at least once weekly at home may be very helpful for early detection of 
proteinuria.