Tacrolimus (FK506) is commonly used in allogeneic stem cell transplantation (SCT) to prevent rejection and graft-versus-host disease. Dosing recommendations are mostly based on studies performed in solid-organ transplant recipients. The package insert recommends a conversion rate of 1:4 when changing from the intravenous to the oral formulation. We retrospectively reviewed tacrolimus levels and toxicity in 32 patients who underwent allogeneic SCT between July 2001 and June 2003 using the package insert recommended conversion rate. All patients started tacrolimus on day -1 at a dose of 0.03 mg/kg actual body weight by continuous IV infusion. Levels were drawn peripherally on day +1, +3, +5 and then biweekly. Target range for tacrolimus levels was 8–12 ng/mL. Conversion from IV to PO at a conversion rate of 1:4 occurred when patients could tolerate P.O. medication. Thirty-two patients underwent alloSCT for a variety of hematologic malignancies. Median age at the time of transplant was 43 year (range 19–58), 78% underwent a MRD donor SCT, 3% a 5/6 MRD SCT, and 19% a MUD SCT, conditioning was Cy/TBI (89%) or Bu/Cy2 (11%). Ninety-three percent of patients with available data required at least one IV dose reduction, the median day of IV to PO conversion was +11 (range +7–+25). The median initial PO dose was 0.08 mg/kg (range 0.04–0.18) and the median conversion ratio was 0.26 (range 0.12–0.4). Median initial tacrolimus level after conversion was 16.9 ng/mL (range 5.3–52.4). Twenty-seven patients (93%) required a reduction in their PO dosing, only two patients immediately achieved a stable PO dose (defined as two consecutive tacrolimus levels within the therapeutic range). Nine patients (31%) developed a creatinine level greater than twice their baseline level, eleven patients (38%) required intervention to lower potassium, and three patients (10%) developed tremors. No irreversible complications were encountered. We conclude that the presently recommended conversion rate frequently results in toxic serum levels associated with mild renal insufficiency and other adverse effects. We have changed our conversion rate from 1:4 to 1:2.