A sero-epidemiological survey, using an indirect immunofluorescence test for IgG against Coxiella burnetii (phase II), was carried out in the Netherlands. Serum samples taken in 1968, 1975, 1979 and 1983 were tested. Occupational groups with a supposedly high risk of infection (veterinarians, residents of dairy farms and taxidermists) showed a significantly higher percentage of seropositives than defined controls. The percentage of seropositive amateur wool spinners was significantly higher than that of the controls from the same region. Since 1968 there has been no increase in the percentage of infected persons, indicating that, contrary to earlier assumptions, Q fever has been endemic in The Netherlands for a long time already. The increase in numbers of notified cases of overt Q fever is considered to be the result of the recent introduction of a sensitive indirect immunofluorescence test for IgM antibodies against C. burnetii. Antibody percentages in all age classes between 1 and 64 years were much alike, suggesting that most infections occur in early childhood. This is in accordance with the finding that 35% of our patients are younger than 3 years. The possibility of infection related to childbirth and lactation is discussed.