In an effort to devise a rational protocol for the follow-up of patients who have had destructive treatment for cervical intraepithelial neoplasia (CIN), an analysis has been performed of treatment failures following laser therapy in 2130 women treated between 1980-1989. Of this group, 1253 women have been followed up for 3 years or more and 380 for 6 years or more. There have been 119 treatment failures (5.6%), of which 71% of the women had the second lesion detected during the first year of follow-up, 24% during the second year, 3.3% during the third year, and 1.7% during the sixth year. In 18% of these 119, the second lesion was detected colposcopically in the presence of negative cytology. There were two cases of microinvasion and one frankly invasive lesion following laser, all diagnosed during the second year of follow-up. Our data suggest that if the first year of follow-up includes at least one colposcopic examination, some second lesions will be diagnosed in the presence of negative cytology.