Subjects, who were attending a city-county hospital for obstetrical care, were consecutively sampled according to three age categories -- 13-14, 15-16, and 17-18 years. They responded anonymously to a questionnaire administered by the clinic instructor while they were waiting for their routine prenatal examination. Questions covered personal demographic information, birth control knowledge and use, frequency of intercourse, and perception of risk. The questionnaire was followed by a short five-question hypothetical risk-assessment exercise. Perception of risk was not associated with age, frequency of intercourse, or level of fertility knowledge. Sexually active adolescents were surprised at their pregnancy. They were generally unable to use concepts of risktaking, even after concrete examples and teaching techniques had been presented. These findings suggest that standard contraceptive motivational and teaching techniques have little relevance to this group of adolescents. Clinicians must use creative, concrete techniques of contraceptive and reproductive instruction that are sensitive to stages of cognitive development. 6 tables, 15 references. (Author abstract modified)