Sexual activity among unmarried adolescents is a major public health problem in Nigeria, because unmarried pregnant girls are more likely to have multiple sex partners and are less likely to use contraceptives, putting them at greater risk for sexually transmitted diseases (STD), unplanned pregnancy, abortion, social isolation, and poverty. Teen pregnancy and STD rates are on the rise in Nigeria, yet few data exist on the experience of the adolescents themselves. This phenomenological study was designed to explore the in-depth experiences of 10 pregnant, unmarried adolescent girls aged 16-19, including the factors contributing to their sexual activity. An ecological model served as the conceptual framework to permit individual experiences to be understood in their social and ecological context. Semistructured interviews and Hycner's method of analysis were used to collect and analyze the data. Results showed that the decision to initiate sexual activity among these girls was influenced by many factors, including: the need for financial support and a socially condoned system of "sugar daddies" who support girls in return for sex; peer pressure to have a sex partner; a romantic knowledge of sexual behavior based primarily on the mass media; and inadequate sex education. As a result of their pregnancy, the girls experienced negative reactions from their families and community, and serious psychological and financial concerns about their prospects for future marriage and their child's identity. A comprehensive community-based reproductive health program is called for, with reliable sex education, cooperation from the mass media, and support from family and community members. The social change implication of this study is to potentially lead to a decrease in unplanned pregnancy, STDs, social isolation, and poverty among adolescent girls in Nigeria.