Losses of heterozygosity on the short arm of chromosome 3p are common in cervical carcinomas in the 3p13-3p21 region, and can be observed in intra-epithelial lesions accompanying cervical cancers. As a preliminary attempt to determine whether these losses can be observed in intra-epithelial cervical lesions without concomitant invasive carcinoma, we have used two microsatellite markers located at the two most frequently deleted segments of the 3p13-3p21 region. We have studied 36 cases of grade II and grade III cervical intra-epithelial neoplasias obtained by conisation biopsies and 30 cases of cervical carcinoma including 3 micro-invasive squamous cell carcinomas. We found loss of heterozygosity or microsatellite instability in 6 of 16 (38%) and 9 of 23 (39%) informative cases of cervical carcinoma at 3p13 and 3p21, respectively. Four of 27 (15%) cases of cervical intra-epithelial neoplasia showed loss of heterozygosity at 3p13, whereas loss of heterozygosity or microsatellite instability at 3p21 was found in 5 of 19 cases (26%). No relationship was found between 3p loss of heterozygosity and human papillomavirus infection. In conclusion, losses of heterozygosity at 3p13 and 3p21 occur in premalignant lesions without concomitant invasive lesions. The prevalence and precise extent of these losses should be established by a more extensive analysis.