Chlamydia pneumoniae infections may spread subclinically. The present investigation took place in a military setting. Sera drawn when the conscripts had entered their military service 2 months previously had been kept frozen and were available. In a camp with 500 people, 35 (7%) developed clinical symptoms of pneumonia. The infection was serologically verified with C. pneumoniae-specific micro-immunofluorescence technique. Of 40 healthy controls, 21 turned out to fulfil the serological criteria of infection, thus, representing subclinical cases. These 21 cases, when extrapolated to the whole camp, equalled a rate of 49% which, added to the 7% of pneumonic cases, gave a total infection rate of 56%. Pre-existing IgG antibodies were demonstrated in 10% of the pneumonic cases, 48% of the subclinical cases, and 89% of the non-infected, healthy controls. Without the access to pre-epidemic sera permitting us to establish 4-fold titre rises, the spread of subclinical C. pneumoniae infection would have been noted at 5%, and not 49% as here demonstrated.