Colposcopy and biopsy were carried out at diagnosis and at follow up of 222 women. Of 322 cervical biopsy specimens taken, 174 were from women with chlamydial cervicitis (patients) and 48 from control women (both at diagnosis) and 100 from 76 patients and seven controls at follow up. Of the 174 patients with chlamydial cervicitis, 158 (91%) had erythema compared with 9/48 (19%) controls, and 140 (81%) had "follicles" and lumps compared with three (6%) controls. The cervical polymorphonuclear leucocyte (PMNL) count in a high power (x 1000) field (HPF) was 85 in patients compared with 47/HPF in controls. Cervical ectopia was found in 154 (89%) patients compared with 32 (67%) controls. After treating the 174 patients, we found erythema in nine (5%) and "follicles" in 16 (9%); both conditions were disappearing. Lymphocytic germinal follicles were found on histology in only 5/165 patients compared with none in controls or in patients at follow up examination (after treatment for chlamydial cervicitis). Inclusions were found in 6/165 patients compared with none in controls or patients at follow up. Chlamydiae were found on electron microscopy in slides from seven out of 159 patients compared with none from controls or 81 subjects at follow up. Estimating numbers of inflammatory cells and measuring vascularity showed diffuse increases in lymphocytes and plasma cells and increases in vascularity in both endocervical and exocervical tissue of patients with chlamydial cervicitis. These increases were lessened by treatment.