Stearic and iodostearic acid inhibit growth of a cervical carcinoma cell line in vitro. This study was performed to determine if iodostearic acid would induce regression of cervical intraepithelial neoplasia (CIN). Women with histologically-proven CIN II or III were randomised into two groups. Those in the first group were given pessaries composed of iodostearic acid in polyethylene glycol (PEG) base. Women in the second group were given pessaries containing only the PEG base. One pessary was inserted into the vagina nightly for 30 nights, and each woman then had the CIN lesion removed by CO2 laser cone excision. There was no difference in the histology of the cone biopsies between the groups, demonstrating that this regime of iodostearic acid has no useful role in the treatment of CIN II-III.