PNEUMONIA WITH SEROLOGICAL EVIDENCE OF ACUTE INFECTION WITH THE CHLAMYDIA-LIKE MICROORGANISM-Z

''Z'' is a recently discovered microorganism that may belong to a new genus in the family Chlamydiaceae. Using an ELISA test we developed, w e measured levels of serum antibody against ''Z'' for 308 paired sera obtained from adult patients hospitalized with community-acquired pneu monia (CAP). In 114 patients (37%), serological evidence of past infec tion with ''Z'' was found. In eight patients (2.6%) there was serologi cal evidence of acute infection with this pathogen. In four of these e ight patients, no other pathogen for CAP was identified despite an int ensive serological investigation encompassing 13 etiological agents. T he four patients were about 30 yr old, and three of them had no histor y of chronic illness. Their illness was characterized by high fever, a nonproductive cough, gastrointestinal symptoms, a shift to the left i n the white blood cell count, and a prompt, dramatic response to eryth romycin therapy. We conclude that the microorganism ''Z'', or a close variant, is infectious for humans, in some cases causing CAP. In these cases the disease is mild and responds quickly to treatment with eryt hromycin.