This paper calls attention to the impact of masculinity ideology, an aspect of gender-related attitudes, on adolescent males' heterosexual relationships. Previous approaches to the male gender role and close relationships, and attitudes toward the male gender role (the operationalization of masculinity ideology), are briefly reviewed. Data from the 1988 National Survey of Adolescent Males are reported. With sociodemographic and personal background factors controlled, males who hold traditional attitudes toward masculinity indicate having more sexual partners in the last year, a less intimate relationship at last intercourse with the current partner, and greater belief that relationships between women and men are adversarial—characteristics suggesting less intimacy in their heterosexual relationships. They also report less consistent use of condoms, specific attitudes about condoms associated with low condom use, less belief in male responsibility to prevent pregnancy, and greater belief that pregnancy validates masculinity. These associations persist when more global gender role attitudes are controlled. Traditional masculinity ideology is thus associated with characteristics suggesting limitations in the quality of adolescent males' close heterosexual relationships, and increased risk of unintended pregnancy and sexually transmitted diseases, including AIDS.