The role of vaginal cytology screening in preventing invasive carcinoma of the cervix was investigated in Cali, Colombia. The history of previous participation in screening programs was obtained from 204 patients with invasive carcinoma and from 2 sets of age-matched controls: one from the same health center and one for the same neighborhood of the patient. Our results show that the risk of developing invasive carcinoma is at least 10 times greater in non-screened than in screened women.