During a recent epidemic of Plasmodium falciparum malaria in Hlabisa district, KwaZulu Natal, polyuria was recognized in a subset of adults presenting to hospital with severe disease. In January to May 2000, following consent, cases of severe malaria provided blood spots for HIV testing, clinical data, and blood and urine samples for investigation of the polyuria. Four hundred and eleven adults with malaria were admitted, of whom 175 had severe malaria and 37 had polyuria (21% of severe cases). In those with polyuria the mean 24-h urine output was 3018 ml or 2.85 ml/kg/h. Ten of 15 (66%) patients investigated had urine and plasma osmolalities consistent with diabetes insipidus. There was no significant association between polyuria and sex, age, HIV status or features of severe malaria. Whether the polyuria in these non-immune adults is a localized problem unrelated to malaria, or a hitherto unrecognized complication of severe malaria is not clear. Physicians are alerted to this observation, and the need for more clinical and pathophysiological studies.