Vitamin D deficiency, which is a recognized problem in haemodialysis (HD) patients, has been associated with higher all-cause mortality. There are no guidelines concerning vitamin D supplementation in HD patients. This study aimed to assess the effects of once-monthly supplementation with high-dose cholecalciferol (vitamin D(3)) in HD patients. Patients with 25-hydroxy vitamin D (25[OH]D) levels of < 75 nmol/l received 40,000 IU of cholecalciferol oncemonthly for 3 months in succession. Every 4 months, 25(OH)D levels were measured and, based on the findings, cholecalciferol therapy was continued for another cycle if necessary. Six cycles were completed in the 24-month study period. The majority of HD patients had mild or severe vitamin D deficiency at baseline. Monthly supplementation with cholecalciferol at 40,000 IU was well tolerated, safe and inexpensive. The treatment regime was effective for vitamin D insufficiency but did not prove to be enough to restore 25(OH)D levels in HD patients with mild or severe vitamin D deficiency.