Of 11 young women with acute peritonitis proved by laparoscopy, seven of whom also had perihepatitis, nine had serological evidence of recent infection with Chlamydia trachomatis; In five of these nine patients high antibody titres to chlamydiae were found without laboratory evidence of gonococcal infection, while the other four had evidence of simultaneous gonococcal infection. C trachomatis may play an important part in peritoneal inflammation previously attributed only to gonococci.