In a study of 41 consecutive patients with bacteraemia-associated community-acquired lobar pneumonia due to Streptococcus pneumoniae and Klebsiella pneumoniae an attempt was made to determine whether distinguishing criteria of disease due to these organisms could be identified according to demographic features and the results of initial clinical and laboratory investigations. Such information would aid in the early initiation of appropriate antimicrobial therapy. The most significant difference between the two groups of patients was the lower platelet count in the K. pneumoniae group (p less than 0.005). In addition leucopenia (p less than 0.05), higher serum albumin (p less than 0.05), and the male sex (p less than 0.05) featured with an increased frequency in patients with pulmonary infection due to K. pneumoniae. Initial antimicrobial therapy in critically ill patients with community-acquired lobar pneumonia and thrombocytopenia, particularly when associated with leucopenia and in male patients, should include agents effective against K. pneumoniae.