In a controlled trial, a hypotonic oral rehydration solution (ORS) (Na+67, K+20, Cl-66, citrate 7, glucose 89 mmol/l osmolality 249 mosmol/kg) was compared with a standard WHO-ORS (Na+90, K+20, Cl-80, citrate 10, glucose 111 mmol/l, osmolality 311 mosmol/kg) in 60 children aged 5-24 months with acute watery diarrhoea. In the hypotonic ORS group, stool frequency, proportion of children who vomited, ORS requirements and purging rate over 24-48 h were reduced by 33% (p = 0.01), 30% (p = 0.02), 21% (p = 0.067) and 21% (p = 0.03), respectively. The proportion of children who vomited and the purging rate over 48 h were reduced by 23% (p = 0.03) and 10% (p = 0.097), respectively. Serum electrolytes after 48 h were comparable. The beneficial effect of hypotonic ORS was most marked in, and largely contributed by, the subgroup negative for rotavirus.