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, we measured levels of serum antibody against ''Z'' for 308 paired sera obtained from adult patients hospitalized with community-acquired pneumonia (CAP). In 114 patients (37%), serological evidence of past infection with ''Z'' was found. In eight patients (2.6%) there was serological evidence of acute infection with this pathogen. In four of these eight patients, no other pathogen for CAP was identified despite an intensive serological investigation encompassing 13 etiological agents. The four patients were about 30 yr old, and three of them had no history of chronic illness. Their illness was characterized by high fever, a nonproductive cough, gastrointestinal symptoms, a shift to the left in the white blood cell count, and a prompt, dramatic response to erythromycin 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 erythromycin.