Abstract Introduction MR-4, the new oral formulation of tacrolimus that allows once-daily dosing, may improve patient compliance. The purpose of this study was to evaluate the safety and efficacy parameters among a group of stable renal allografts after conversion to MR-4. Methods We enrolled 82 stable kidney recipients, who had received their grafts 43.9 ± 38.3 months prior. They were of mean age 56 ± 12 years and included 70.7% men. Sixty-six patients were converted on a milligram-for-milligram basis from their total daily dose; the remaining patients were converted at the physician's discretion. Three patients were excluded: 1 because of the development of abdominal pain, and 2 because of dosing errors. Tacrolimus trough levels and renal function tests were evaluated at entry and on days 7, 30, and 90. Results Only 5 (7.6%) converted patients required a later dose adjustment. In the group of 61 patients who did not require this adjustment, the mean tacrolimus trough levels decreased during the first week (6.8 ± 1.7 to 5.8 ± 2.0; P < .000). Thirty-eight patients completed 3 months of follow-up. Their tacrolimus trough levels, serum creatinine levels, and proteinuria remained stable. The number of capsules per patient needed after the conversion to MR-4 was lower (3.9 ± 1.6 versus 2.9 ± 1.0; P < .000). There were no cases of acute rejection episodes. Conclusion Based on a milligram-for-milligram conversion, only 7.6% of our patients required a dose adjustment. With this conversion, an initial decrease in tacrolimus trough levels was documented at day 7, which remained stable to the end of the study. The patients needed a lower number of capsules. These results supported the safety of MR-4.