Eleven patients with a serum phosphate concentration below 0.3 mmol/l were detected in a prospective study of severe hypophosphatemia carried out over 7 weeks on 13 579 adult inpatients. For only one of these patients had a serum phosphate determination been requested by the ward, whereas the remaining cases of severe hypophosphatemia were detected as a result of the study. The most common cause of unexpected severe hypophosphatemia was chronic alcoholism.