Roxithromycin, a new macrolide, has favorable pharmacological properties: it is acid stable and well absorbed, and its prolonged half-life allows b.i.d. prescription of 150 mg. In our open prospective study we found an etiologic infectious agent in 16 (80%) of 20 patients presenting with "atypical" pneumonia. Ten of the 16 pneumonias were due to an organism sensitive to roxithromycin (gram+ cocci, gram- rods, mycoplasma and chlamydia), and 6 to a virus. Therapeutic success was obtained in greater than or equal to 90%. The treatment was well tolerated with fewer gastrointestinal disorders than with erythromycin.