This study was undertaken to determine factors influencing the success of loop excisions. One hundred twenty-seven women with exocervical intraepithelial lesions underwent large loop excision of the transformation zone (LLETZ); 102 returned for evaluation at 3 months. The pretreatment biopsies were low grade squamous intraepithelial lesions (LGSIL) in 37 cases and high grade squamous intraepithelial lesions (HGSIL) in 90. For the 37 women with LGSIL, the LLETZ specimen revealed no residual SIL in 13 (35%), LGSIL in 16 (43%), and HGSIL in 8 (22%). For the 90 women with HGSIL on pretreatment biopsy, 17 (19%) had no SIL, 10 (11%) had LGSIL, and 63 (70%) had HGSIL. Of the 102 women who returned for reevaluation, colposcopy was satisfactory in 89. There were 9 failures and all of these occurred in women with HGSIL in the LLETZ specimen. In the 17 women with involved margins there were 6 failures (35%); in the 85 women with uninvolved margins there were 3 failures (4%) (p = 0.005). The success of loop excisions is influenced by the grade of intraepithelial neoplasia and status of the margins of the LLETZ specimen.