The prevention and/or suppression of preneoplastic foci in animals has been convincingly demonstrated with the use of a variety of compounds, including retinoids. We report here a phase I/II trial of beta-trans-retinoic acid delivered via a collagen sponge/diaphragm insert for neoplasia of the cervix. Eighteen patients with biopsy-proved grades 2 and 3 cervical intraepithelial neoplasia underwent four consecutive daily applications of trans-retinoic aicd to the cervix. Conization of the cervix was performed 4 weeks later. Significant vaginal toxicity in 10 of 18 (55%) patients was unrelated to the concentration or carrying media of the drug. A reduction in size of the intraepithelial lesion was noted by colposcopy in six of 18 (33%) patients with complete resolution of disease at conization in two patients. A new delivery system designed to avoid vaginal toxicity while defining the optimal concentration and dosing schedule for these patients is currently under investigation.