A prospective study on consecutively hospitalized pneumonia patients showed that 41.5% of 58 patients had a fourfold rise in the complement-fixation titer for Mycoplasma pneumoniae. Viral isolation techniques and serologic tests for influenza A1, A2 and B, para-influenza 1 and 3, respiratory syncytial virus and the adeno virus group yielded only a single positive isolate for influenza A2. Serologic tests for melioidosis, leptospirosis, scrub, murine and epidemic typhus and psittacosis were all negative. The clinical manifestations were not distinctive for the positive M. pneumoniae patients when compared with the patients having a negative M. pneumoniae complement-fixation test. The symptoms and signs and laboratory and radiologic findings were similar to those described in other reports on primary atypical pneumonia.