The Netherlands experienced an unprecedented outbreak of Q fever between 2007 and 2010. The Jeroen Bosch Hospital (JBH) in 's-Hertogenbosch is located in the centre of the epidemic area. Based on Q fever screening programmes, seroprevalence of IgG phase II antibodies to Coxiella burnetii in the JBH catchment area was 10·7% [785 tested, 84 seropositive, 95% confidence interval (CI) 8·5-12·9]. Seroprevalence appeared not to be influenced by age, gender or area of residence. Extrapolating these data, an estimated 40 600 persons (95% CI 32 200-48 900) in the JBH catchment area have been infected by C. burnetii and are, therefore, potentially at risk for chronic Q fever. This figure by far exceeds the nationwide number of notified symptomatic acute Q fever patients and illustrates the magnitude of the Dutch Q fever outbreak. Clinicians in epidemic Q fever areas should be alert for chronic Q fever, even if no acute Q fever is reported.