Despite the availability of immunosuppressive drugs such as prednisone, cyclophosphamide, cyclosporine A (CyA) and mycophenolate mofetil for the treatment of steroid-dependent idiopathic nephrotic syndrome (SDNS), medication-free remission is not achieved in a number of patients. To avoid excessive steroid toxicity, the use of tacrolimus (Tac) has been discussed. We report on five children diagnosed with SDNS on the histological basis of minimal change glomerulopathy or focal segmental glomerulosclerosis. Following the failure of other medications to achieve sustained remission, Tac was administered to these patients who varied in age from 10.5 to 13.5 years. Only one patient showed a substantial reduction in the number of relapses with the Tac treatment. Two boys, after 9 and 44 months on therapy, respectively, developed insulin-dependent diabetes mellitus (IDDM), necessitating the withdrawal of Tac and the daily use of insulin for 3 and 6 months. In both patients hyperglycemia had occurred during prednisone-based relapse therapy of SDNS. The patients had low serum protein concentrations, presumably increasing the free active Tac fraction, while trough levels of the drug remained unchanged. Both of the affected patients had additional risk factors for impaired glucose tolerance, such as morbid obesity (patient 1; BMI: 41.6 kg/m(2)) and African American origin (patient 2). Our case reports demonstrate that the use of Tac in patients with SDNS may be associated with an increased risk for IDDM, especially during relapse of NS, and particularly if additional risk factors are present. Moreover, Tac does not appear to substantially increase the success of treatment.