Introduction On August 14, 2008, a general practitioner reported four cases of measles within a single family, where a child had attended a summer camp. Contact with camp officials led to the identification of two additional cases. An investigation was launched to document transmission during the camp and, subsequently, within the families of camp participants. Methods A retrospective cohort study was conducted involving all individuals who attended the summer camp from July 12 to 21, 2008, as well as their families. A standardized questionnaire was completed by telephone with the families. Results The questionnaire was completed for 37 of the 40 families, involving 258 individuals. A total of 58 cases of measles were identified: 18 among camp participants and 40 in their families. Only 10 of the 58 identified measles cases were reported by 3 physicians, and 43% of the cases had not consulted a general practitioner. Vaccination coverage was 81% in families with no cases and 19% in families with at least one case of measles. None of the cases had been vaccinated, and the attack rate among susceptible individuals (unvaccinated and with no history of measles) was 92%. Three initial cases were directly or indirectly responsible for 11 measles cases in the camp and 44 additional cases in the families. These index cases were attending school in June at schools where measles cases had reportedly occurred. This summer camp and these schools, as well as those where measles outbreaks occurred last June, are managed by the same religious organization. Discussion This investigation illustrates the very high risk of measles virus transmission in an insufficiently vaccinated population. It also confirms the resurgence of virus circulation, which has already been evidenced by several clusters of cases in France since the beginning of the year. (R.A.)