Background: Vitamin D (VitD) deficiency is common following biliopancreatic diversion (BPD). We conducted a prospective open-label study to evaluate the efficacy of a single intramuscular injection with 600,000 IU of cholecalciferol in an arachis oil depot formulation (VitD3, Arachitol Solvay Pharmacia) as an adjunct to regular oral VitD supplementation (Citrical+D) for a period of 12 months following BPD surgery. Methods: Some 29 patients who had undergone BPD during 2000-2005 were recruited and received a single injection of 600,000 IU of cholecalciferol. Venous blood VitD, parathyroid hormone (PTH), alkaline phosphatase (ALP), ionised calcium and urinary N-telopeptide (NTX) were assessed at baseline and at 1.5, 3, 6, 9 and 12 months post-injection. Bone mineral density (BMD) was determined at baseline and 12 months post-injection. Results: VitD concentrations (mean ± SD) were significantly increased from baseline values (61.5±18.8 nmol/L) at 1.5 months (92.4±21.5, p<0.001), 3 months (100.5±24.4, p<0.001) and 6 months (79.1±20.9, p=0.014) post-injection, with non-significant elevations at 9 months (73.3±15.1, p=0.248) and 12 months (73.4±17.3, p=0.278). The proportion of patients with 'normalised' VitD levels was significantly higher at all post-injection time points (range, 93-100%) compared with baseline (71.4%; p<0.01). Ionised calcium and ALP remained within normal levels at baseline and all follow-up time points, although ionised calcium decreased by 3.4% (p=0.015) and ALP increased by 14.6% (p=0.021) at 12 months compared with baseline. No significant change in PTH, NTX or BMD was observed. Conclusions: Intramuscular cholecalciferol injection, as an adjunct to oral supplementation, appears a safe and effective method to increase and maintain VitD levels after BPD.