The first reported case of chikungunya virus diagnosed by serology in Thailand was in 1960 and the last one was in 1991. The disease surveillance system does not specifically include chikungunya cases and the signs and symptoms are similar to these of dengue fever/dengue hemorrhagic fever (DF/DHF), rubella, and fever of unknown origin (FUO); thus cases might often be reported under those diagnoses. During the rainy season of 1995 (Jun-Aug), there were at least 2 reported chikungunya outbreaks which might indicate that it is a re-emerging disease in Thailand. However, there is still limited information and knowledge on some aspects of this disease such as clinical manifestations, subclinical cases, duration of illness, complications, transmission, immunity, and reservoirs. Thus, the objectives of this paper are to describe the epidemiology of chikungunya infection based on outbreak investigations carried out in Khon Kaen (July 1991), Nakorn Si Thammarat (July 1995), and Nong Khai Provinces (August 1995). All three outbreaks occurred during the rainy season. The three most common clinical manifestations were fever with severe althralgia with maculopapular rash. Both sexes and all age groups were affected. Serological results were positive for IgM, with four-fold rises in paired sera, and viral isolation in Nakorn Si Thammarat and Nong Khai. Only in Nong Khai was hemagglutinin inhibition conducted and the results were positive. No deaths were reported. The outbreaks occurred in rural villages and all three larval indices (BI, HI, CI) were very high. The possible vectors in these outbreaks were Aedes aegypti and Aedes albopictus. In the Nong Khai outbreak, blood specimens were taken at the 3-5th day after onset and therefore the proportion of positive results was low. IgM antibody of follow-up cases declined within 3 months, villagers from all three areas with outbreaks mentioned that they had no previous experience of this disease. This suggests that chikungunya infection is a re-emerging disease.