Between June 1982 and May 1983, rotavirus was found in 29% of 248 pediatric patients between 1 month to 5 years of age at Siriraj Hospital, Bangkok, Thailand. Among 413 household contacts, RV was detected in 9 (2%) of whom 6 developed diarrhoea. RV was not found in age-matched healthy controls. In a serological study, the serological response was 81% in rotavirus diarrhoea cases (17 out of 21) by ELISA. By the ELISA blocking test, the seroconversion rate was only 29.6% in rotavirus diarrhoea cases (8 out of 27) and in others, 2 out of 71 cases (2.6%) who also showed serological response did not have rotavirus in their stools. This study indicates role of rotavirus in causing acute diarrhoea in young children, suggests a guideline for diagnosis and management of the patients. The rotavirus immunization should be considered if the vaccine becomes available in the near future.