65 episodes of rotavirus diarrhoea, detected during a longitudinal follow-up of 336 infants from birth to 24-32 months of age, were analyzed for clinical symptoms. Rotavirus gastroenteritis was characterized by watery diarrhoea, vomiting (particularly in older children), fever and dehydration. A 0-20 point numerical score was devised according to the distribution of clinical features in the patients. Using this system, the mean severity score for the 65 episodes of rotavirus diarrhoea was 11.0 +/- 3.7 as compared to 5.6 +/- 3.2 for the 183 episodes of non-rotavirus diarrhoea in the same population (p less than 0.0001, t-test). The 20 point score is proposed for analysis of efficacy studies of candidate rotavirus vaccines.