Simkania negevensis, a recently found Chlamydia-like organism, has been associated with bronchiolitis and pneumonia in children. S. negevensis findings have been common also in healthy, non-symptomatic subjects. Antibodies to S. negevensis were measured by microimmunofluorescence in 174 frozen paired sera obtained from children with community acquired pneumonia in a population-based study. There was evidence of S. negevensis infection in 18 (10%) cases. All diagnoses were based on the presence of specific IgM antibodies. The numbers of S. negevensis cases increased from 2 (4%) at <24 months to 7 (15%) at > or = 10 y of age. 12 (67%) were mixed infections with viruses or other bacteria. 16 children (9%) had measurable IgG antibodies to S. negevensis, but significant rises were not found in any cases. Thus, S. negevensis may be a real, though rare, cause of CAP in children, occurring often in mixed infections with viruses and other bacteria.