The Negative Attitudes toward Masturbation Inventory (NAMI) is one among the scarce self-reports developed to assess attitudes toward masturbation. The present study aims to examine its factorial structure through confirmatory factorial analysis (CFA), but also to analyse its reliability and give evidences about its convergent and discriminant validity. A sample of 1.083 adolescents (13-17 years old) responded to the NAMI, the Sexual Opinion Survey, and the Hurlbert Index of Sexual Fantasies. Exploratory factor analysis yielded three factors, but one of them was eliminated because of its poor psychometric properties. Later CFA supported the resulting two-factor structure, showing appropriate internal consistency values, and good convergent and discriminant validity evidences. © 2013 Fundación AIGLÉ.