There was a large increase in incidence of reported cases of ornithosis in Norway, Sweden, and Denmark in 1981-1983. Serologic evidence indicated that the epidemic was caused by the newly described Chlamydia organism, C. pneumoniae, strain TWAR. When serum samples that had been shown to contain Chlamydia complement fixing antibodies were tested by the microimmunofluorescence method with TWAR antigen, 49%-71% showed evidence of current TWAR infection during the epidemic years compared with 10%-20% in nonepidemic years.