Serological distinction between ornithosis and infection by Chlamydia pneumoniae (strain TWAR) was attempted by microimmunofluorescence (MIF) of 156 cases previously considered as ornithosis on the ground of a positive complement fixation test with a chlamydia group antigen. At least 23% of these cases were TWAR infections according to the results at retesting by MIF. The highest frequency of TWAR infections was demonstrated during 1981 with a decreasing number and proportion thereafter. We contend that the sudden increase in the reported number of ornithosis cases in 1981 may have been caused by TWAR infections which seem to vary in incidence over time. Family clusters of respiratory infections were much more common with TWAR infections than with ornithosis.