We describe 2 cases of erythema nodosum (EN) secondary to an infection with the TWAR strain of chlamydia, recently designated Chlamydia pneumoniae. Two young patients, 17 and 11 years old, were admitted with EN and no physical signs of pneumonia. One patient had a non-productive cough and fever. The other patient only ran a high fever. Chest radiography revealed bronchopneumonias. Infection with the C. pneumoniae species was proven by serologic testing using microimmunofluorescence technique. Serology and cultures for other bacteria known to induce EN were negative. Thus, C. pneumoniae (strain TWAR) can elicit EN.