SummaryOne hundred cases of hypophosphataemia (< 2-0 mg/100 ml) and 84 cases ofhyperphosphataemia (> 5.0 mg/100 ml) occurring in a hospital population were studied in order to determine the cause of the abnormality. Examples of hyperphosphataemia due to renal failure were excluded from the study.A low serum phosphorus concentration was most frequently due to intravenous administration of carbohydrate, usually glucose, which accounted for 40% of cases. The next commonest cause was vomiting (12%). No obvious explanation could be found in 26% of cases, but in most of these factors were present which are known to affect phosphorus metabolism.No one cause of hyperphosphataemia was outstanding in frequency and in over 50% of cases no definite explanation for the abnormality could be found. (Fourman and Royer, 1968;Henry, 1969;Fraser and MacIntyre, 1970;Hoffman, 1970). A recent editorial (Lancet, 1970) stated that "despite the central role of phosphate in the ADP-ATP system and its importance as a major constituent of bone and teeth, as well as a urinary buffer, phosphate homoeostasis has received surprisingly little attention."With the advent of multichannel screening the opportunity has arisen of studying changes in serum phosphorus and their causes in a large number of patients who previously would not have had this test performed. We report here an analysis of 100 cases of hypophosphataemia and 84 cases of hyperphosphataemia encountered in a hospital population. Because of the preponderance of renal failure as a cause of hyperphosphataemia it was decided to exclude patients with this disorder from the study. Outpatients were also excluded as, in general, they could not be satisfactorily followed up.