The objective was to evaluate natural cytotoxicity of peripheral blood during interferon treatment in cervical intraepithelial neoplasia grade 2 (CIN2), as index of interferon activity. Twenty-one patients with CIN2, histologically proven, were treated with interferon α 2b (Intron A, Sheering-Plough Corp.), in a dose of 3,000,000 units three times per week for 8 weeks self-administered by intramuscular injection. The rate of remission to the interferon treatment was 38.1%. Eight healthy patients had a significant increase in natural killer activity during and after the treatment (p < 0.001), whereas the 13 nonresponder patients remained with low values of natural killer activity. By analyzing the basal natural killer activity before the treatment, the patients with clinic pathologic remission had a significantly higher mean value than nonresponder patients (p = 0.007). Notwithstanding the specimen exiguity, the individual basal natural killer activity seems to be a predictive parameter of interferon treatment response in patients with CIN2.