The purpose of this study was to identify the reasons for treatment failures in patients managed with cervical intraepithelial neoplasia who subsequently developed invasive carcinoma of the cervix. Of 672 patients seen with cervical carcinoma from 1980 to 1990 inclusive, at the Tom Baker Cancer Centre, 24 (3.6%) had previously undergone conservative treatment for CIN and represent the current study population. The initial colposcopic-guided biopsy showed metaplasia (2), CIN 2 (5), and CIN 3 (17). The conservative treatment methods included observation (5), electrocautery (1), laser ablation (3), surgical cone (5), and cryotherapy (10). The mean time interval in months from conservative treatment of CIN to diagnosis of cervical cancer was 21.8 with cryotherapy and 26.7 with laser ablation. The FIGO stage of invasive cervical cancer was Stage 1A (7), Stage 1B (15), Stage 2A (1), and Stage 3 (1). The single death was a patient aged 30 with metastatic small cell cervical carcinoma arising within 4 years of cryotherapy for CIN 3. Of the 24 patients, 13 were managed appropriately yet developed carcinoma, 3 deviated from an accepted standard colposcopy protocol, 5 had inadequate follow-up, 2 refused treatment, and 1 developed de novo disease. The principle reason for treatment failure according to the literature is blatant deviation from protocol. This study, however, suggests that established invasive disease may have gone undetected prior to an ablative therapy. Difficulties related to diagnosis are discussed. The importance of peer reviews becomes evident if practices are to be evaluated and changes to protocols are to be implemented.