AIM Hyperphosphataemia is almost inevitable in end stage renal disease (ESRD) patients and is associated with increased morbidity and mortality. In this study we examined whether oral activated charcoal (oAC) reduces serum phosphate level in haemodialysis patients. METHODS This was an open-label, prospective, uncontrolled study. One hundred and thirty-five haemodialysis patients were included in this study, with cessation of treatment with any phosphate binders during a 2 week washout period. Patients with serum phosphate levels greater than 5.5 mg/dL during the washout period were included for treatment with oAC. oAC was started at a dose of 600 mg three times per day with meals and was administered for 24 weeks. oAC dose was titrated up during the 24 week period to achieve phosphate control (3.5-5.5 mg/dL). A second 2 week washout period followed the end of oAC treatment. RESULTS In the 114 patients who successfully completed the trial, the mean dose of activated charcoal was 3190 ± 806 mg/day. oAC reduced mean phosphate levels to below 5.5 mg/dL, with mean decreases of 2.60 ± 0.11 mg/dL (P < 0.01) and 103 (90.4%) of the patients reached the phosphate target. After the second washout period the phosphate levels increased to 7.50 ± 1.03 mg/dL (P < 0.01). Serum intact parathyroid hormone (iPTH) levels declined from 338.75 ± 147.77 pg/mL to 276.51 ± 127.82 pg/mL (P < 0.05) during the study. oAC had no influence on serum prealbumin, total cholesterol, triglycerides, serum ferritin, haemoglobin or platelet levels and the levels of 1,25-dihydroxyvitamin D were stable during the study. CONCLUSION In this open-label uncontrolled study, oAC effectively controls hyperphosphataemia and hyperparathyroidism in haemodialysis patients. The safety and efficacy of oAC needs to be assessed in a randomized controlled trial.