Forty-two patients were entered into a phase I trial to evaluate the vitamin A derivative, trans-retinoic acid, in cervical intraepithelial neoplasia. Treatment consisted of four consecutive 24-h applications of retinoids via an inert collagen sponge in a cervical cap. Patients were followed for response at 3-month intervals using cytology, colposcopy, and selected biopsies. Thirty-six patients were evaluable (mild dysplasia, 13; moderate dysplasia, 17; severe dysplasia, 6) with follow-up from 5 to 18 months. Complete regression was seen in 2/14 (14%) patients treated with concentrations of 0.05%----0.1167% and in 10/22 (45%) patients treated with concentrations of 0.1583%----0.484% (p less than 0.05). One patient with negative biopsies at 12 months has subsequently recurred at 18 months.