We report an epidemiological investigation of an explosive common-source water-borne shigellosis outbreak traced to a piped public water system in northern Thai communities. In August 1991, 242 cases of acute diarrhoeal illness occurred in Sam Ngao Subdistrict, Tak Province. About 30% of the cases were culture-positive for multiresistant Shigella flexneri 2a. The outbreak affected all age groups, with the highest attack rate (9.4%) in children < 5 years old. The first ten cases occurred during 1 and 5 August 1991 and a subsequent 158 cases (65.3%) clustered between 6 and 10 August 1991. Most cases (93.0%) occurred in the villages sharing the common piped water system A. The inhabitants who were served by system A had a significantly (P < 0.01) higher attack rate of infection (7.0%) than those who used the other piped public water system B (0.1%) or well water (0.3%). A case-control study revealed a significant association between disease and drinking unboiled piped water (P < 0.05, odds ratio 2.8). The implication of piped water was supported by the presence of faecal contamination in the piped water system, the result of interrupted chlorination. Rapid identification of the possible transmission source and prompt implementation of control measures curtailed the spread of this outbreak.