Two hundred and forty‐six patients with acute respiratory illness and 68 control patients, hospitalized in Copenhagen between March 1966 and March 1968, were examined. By isolation procedures and/or by tests for specific antibodies and cold agglutinins, 2 per cent of the patients with pneumonia were found to be infected withMycoplasma pneumoniaein the first year, while the percentage in the following year was 18. This variation in incidence was also found on examination of sera from patients from the whole of Denmark hospitalized during and beyond the same period. No seasonal variation in the incidence was observed.Mycoplasma pneumoniaeinfection was diagnosed exclusively in patients suffering from pneumonia. Mycoplasmas other thanMycoplasma pneumoniaewere found to be without aetiological significance; they were isolated most frequently from adult