An investigation of patients with primary atypical pneumonia (PAP) was undertaken at a large military base, Camp Lejeune, NC, to determine the cause. Serologic and isolation methods were used to detect viral and Mycoplasma infections. Recovery of virus was the most precise index of adenovirus infection under epidemic conditions. Serologic conversion was the most efficient method for detecting Mycoplasma pneumoniae infection. These studies revealed that adenovirus and M pneumoniae infections were individually associated with PAP simultaneously. Together, these agents were responsible for over 70% of the PAP seen in a three-month period of intensive study. A 30-month survey of pneumonia patients revealed infection with each agent to be associated with about 20% of the PAP sampled. M pneumoniae infection did not exhibit a seasonal predilection while pneumonia associated with adenovirus infection occurred predominantly during the winter months. The finding that more than one agent may be an important cause of PAP in the same population during the same period of time must be considered in future studies of PAP.