End of May 2003 the health department of Soest (HDS), Germany, was notified by a local hospital of an increase of patients with atypical pneumonia. An outbreak investigation was conducted to identify the etiologic agent, extent and source. The first case-control study (case-base design) was conducted to see whether attending a farmers' market where a sheep was lambing, was positively correlated with disease or whether another source was possible. It was conducted with clinical cases registered in the town of Soest and population-based controls from the same town regardless the disease status. In a second case-control study risk factors in the farmers' market were investigated. Therefore only cases and controls from Soest that had attended the farmers market were enrolled. The infectious agent Coxiella burnetii was detected by laboratory testing of patients and the ewe. 299 outbreak-associated cases were reported nation-wide with the majority of cases from the district of Soest. Exploratory interviews led to the hypothesis that a sheep that had given birth to a lamb during a farmers' market on May 4 could be the source. Blood samples confirmed the diagnosis. In the first case-control study, visitors to the farmers' market were 210 times (95%CI=22-3601) more likely than non-visitors to develop Q fever. In the second case-control study the time spend at the gate (OR=17, 95%CI=3,0-112,54) and close distance to the sheep (OR=5, 95%CI=1,[17][18][19][20][21][22]34) were significantly associated with illness. The outbreak was associated with close proximity to a sheep that had given birth to a lamb. We recommend that pregnant ewes in the latter third of their pregnancy are banned from display in the public and that sheep that have close contact with the public are tested for Coxiella burnetii.