The purpose of this study was to assess retrospectively the prevalence of severe hyponatraemia in a hospital population and its laboratory investigation, treatment and clinical outcome. Over a 6-month period 47 patients (27 women and 20 men) were found to have a plasma sodium concentration of < or = 120 mmol/L (this number made up less than 0.17% of all plasma sodium requests over that time period). The mean patient age was 75 +/- 16 years and the average hospital stay was 37 +/- 45 days (1-179 days). Patient mortality was 51% (women 57% and men 43%). The mean initial plasma sodium concentration was 116 +/- 4.5 mmol/L, rising after therapeutic intervention to a mean of 130 +/- 4.2 mmol/L. The mean plasma sodium correction rate was 4.7 +/- 4.3 mmol/L/24 h (0.9-17.5 range). Twelve per cent of the patients had their plasma sodium raised at a rate of greater than 10 mmol/L/24 h after their initial presentation. Two patients may have had symptoms and signs suggestive of cerebral oedema/cortical dysfunction: in one patient the sodium concentration was raised at a rate of 9.5 mmol/L/24 h and in the other at 12.0 mmol/L/24 h. Sixty-one per cent of the patients had a chest infection, 44% were on diuretics, 28% had congestive cardiac failure, 28% were post-operative (9% orthopaedic procedures), 19% had carcinoma and 9% were on a selective serotonin re-uptake inhibitor. Regarding laboratory investigations, 56% had liver function tests, 41% had thyroid function tests, 36% had plasma osmolality determination, 36% had urinary electrolytes including urinary osmolality and < 2% had tests to exclude hypoadrenalism.