Following a study in which the etiology of nearly 70% of 142 cases of pneumonia in children could be determined using a combination of bacteriological and serological methods, the effect of erythromycin ethylsuccinate was compared with that of amoxicillin in a randomized study on 120 cases of pneumonia. We first examined the tracheal secretion microbiologically and determined other serological parameters and clinical data. The tracheal secretion was sterile in only 19% of the cases. We were able to identify the etiology in 64% of the cases using a combination of microbiological and serological methods. A discontinuation of therapy and acceptable side-effects were considerably more frequent with amoxicillin than with erythromycin ethylsuccinate (75 mg/kg body weight). The advantages of erythromycin, especially for the initial therapy of pneumonia, and the improvements in diagnosis resulting from the examination of the tracheal secretion will be discussed.