The occurrence of the nephrotic syndrome in 3 patients treated for 1–8 months with 2-mercapto-propionyl-glycine (Thiola) (at doses ranging from 0.750 to 2.000 g/day) is reported. The drug had been administered for chronic active hepatitis in 1 patient and for cystinuria in the remaining patients. Percutaneous renal biopsies, obtained in all patients and examined at light microscopy, immunofluorescence and electron microscopy, showed ‘minimal change’ nephropathy in 2 patients and membranous glomerulonephritis in the 3rd patient. In all patients, complete remission of the nephrotic syndrome and, eventually, disappearance of proteinuria were observed 15–120 days after discontinuation of Thiola. It is suggested that this drug may act either through an antigenie mechanism, akin to that of penicillamine, or through a sensitization mechanism, both of them being able to cause the nephrotic syndrome.