The role of Chlamydia trachomatis in pelvic inflammatory disease (PID) diagnosed without laparoscopy was assessed by measuring antichlamydial antibodies in the patient's serum and by comparing the results with those in patients with uncomplicated non-specific genital infection (NSGI) and gonorrhoea and in non-infected controls. A modified microimmunofluorescence test was used. Patients with severe PID had significantly more positive antichlamydial IgG and IgM results than did control subjects, patients with gonorrhoea, and patients with NSGI. Less severe PID was associated with significantly raised levels of antichlamydial IgG antibodies compared with NSGI and controls and with raised levels of IgM antibodies compared with controls. Two patients with PID had lower genital tract gonorrhoea, one of whom had raised antichlamydial antibody levels. These findings may indicate a mixed infection and therapy should be reviewed in such patients. A serological diagnosis of chlamydial infection is relatively easy and cheap and enables a rapid diagnosis of chlamydial infection to be made.