Q fever is an acute febrile illness due to Coxiella burnetii. In the Balkans, Q fever in humans has been reported since World War II, and in countries such as Bulgaria the number of cases has increased since the early 1990s. We report an investigation of an outbreak in the town of Botevgrad, Western Bulgaria. Overall, 220 cases were identified between May 1 and June 9, 2004. Of the cases, 168 were from Botevgrad; the others were from neighbouring towns. This has been the largest outbreak in Bulgaria in the last 20 years. Q fever outbreaks in urban areas are not common. Flocks of sheep and goats were the most likely source of infection, as suggested by the observation that flocks grazed in, or had travelled on, the roads and the gardens of the town, and for the prevalence of anti-C. burnetii antibodies among animals in the area. This large outbreak highlights how zoonoses such as Q fever may represent a public-health threat also for urban populations.