Overall 1,261 cases of Q fever were diagnosed between 1984 and 2004 in Gipuzkoa (Basque Country, Spain). Most (75.5%) of the cases ocurred in subjects 15-45 years of age. A total of 79.5% of the cases (n = 1003) ocurred between January and June. The annual incidence for acute Q fever in Gipuzkoa was 7.7, 15.8, 9.6, and 5.7 for the periods 1984-1989, 1990-1994, 1995-1999, and 2000-2004, respectively. In 94% of the cases IgM titer was >/=1/256. The most frequent clinical manifestation was pneumonia (79%). Only two cases of chronic Q fever were detected during the 21 years studied.