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Chikungunya virus (CHIKV) is known to cause sporadic or explosive outbreaks. However, little is known about the endemic transmission of CHIKV. To ascertain the endemic occurrence of CHIKV transmission, we tested blood samples from patients with a non-dengue febrile illness who participated in a prospective cohort study of factory workers in Bandung, Indonesia. From August 2000 to June 2004, and September 2006 to April 2008, 1901 febrile episodes occurred and 231 (12.2%) dengue cases were identified. The remaining febrile cases were evaluated for possible CHIKV infection by measuring anti-CHIKV IgM and IgG antibodies in acute and convalescent samples. Acute samples of serologically positive cases were subsequently tested for the presence of CHIKV RNA by RT-PCR and/or virus isolation. A total of 135 (7.1%) CHIKV infections were identified, providing an incidence rate of 10.1/1,000 person years. CHIKV infections were identified all year round and tended to increase during the rainy season (January to March). Severe illness was not found and severe arthralgia was not a prominently reported symptom. Serial post-illness samples from nine cases were tested to obtain a kinetic picture of IgM and IgG anti-CHIKV antibodies. Anti-CHIKV IgM antibodies were persistently detected in high titers for approximately one year. Three patients demonstrated evidence of possible sequential CHIKV infections. The high incidence rate and continuous chikungunya cases in this adult cohort suggests that CHIKV is endemically transmitted in Bandung. Further characterization of the circulating strains and surveillance in larger areas are needed to better understand CHIKV epidemiology in Indonesia. Author Summary: Chikungunya is one of the neglected diseases. It has only attracted attention during outbreaks, in particular, the large epidemics in the Indian Ocean in 2005–2006. To our knowledge, there has never been any surveillance to determine the transmission of this virus among humans in non-outbreak settings. Such surveillance is particularly important because it will provide a better estimate of the disease burden and valuable information on how this virus is maintained outside outbreaks. Our study, conducted between 2000 and 2008 in Bandung, West Java, Indonesia, yielded several important findings: 1. Chikungunya is an important cause of fever among adults in Bandung, Indonesia. 2. The clinical symptoms are mostly mild and short lasting. 3. In addition to previously described epidemiological features involving episodic outbreaks, it is also continuously transmitted throughout the year. 4. A few patients may have experienced more than one chikungunya virus infection. 5. Only the Asian genotype was found and not the East Central South African genotype that was responsible for the 2005 outbreak in the Indian Ocean. 6. The persistence of IgM for a long period after illness may complicate the interpretation of laboratory results. [ABSTRACT FROM AUTHOR]