We studied serum sodium, plasma osmolality and urinary sodium and osmolality on days 1, 3 and 5 of hospitalization of 100 children aged from 1 month to 12 years admitted with a diagnosis of pneumonia. Hyponatraemia (serum sodium concentration < or = 130 mmol/l) was found in 31 patients at the time of admission. The probable cause of hyponatraemia in 94% of cases was the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Symptoms and signs indicative of severe pneumonia were two to three times more frequent and the mean duration of tachypnoea, chest-wall retraction and hospital stay about one and a half times longer in children with hyponatraemia. Four children died (two on day 1, one on day 5 and one on day 8); all four had a serum sodium concentration < or = 125 mmol/l which persisted until death. Of the remaining 27 hyponatraemic children, serum sodium concentrations returned to normal on day 3 in 26, while in one hyponatraemia persisted until day 7. The recovery from hyponatraemia showed a good correlation with improvement in clinical signs of respiratory distress. The SIADH occurred in about one-third of the children hospitalized for pneumonia, and was associated with a more severe disease and a poorer outcome. Perhaps fluid restriction in these cases may improve the outcome.