Adolescent HIV prevention relies upon accurate assessment of risk, which cross-sectional instruments typically address at a single point in time, offering limited information about dynamic and complex behaviors. Using a Sexual Health History Calendar (SHHC), this study investigates ranges of individual risk taking, variability between-and within-individuals, and patterns of risk taking over 12 months with a clinic-based convenience sample of racially diverse adolescents (N = 232). Monthly sexual risk was coded from 0 (abstinent) to 5 (no condoms + multiple partners). Repeated-measures ANOVA partitioned total sex risk variance into 3 components: between-participant, within-participant (month), and participant-by-month interaction. Average sex risk scores increased significantly over time from 1.38 to 1.84 (β = 0.05, t [ 2734] = 6.47, p < .01); youth with same-sex partners reported wider ranges of risk taking than opposite-sex partnered youth (t  = 1.974, p = .05). Five risk patterns emerged between youth: increasing risk (positive-sloped; 34%), decreasing risk (negative-sloped; 12%), and 3 variations of flat risk (19% consistently low-risk, 18% moderate, 16% high). Variance between-and within-participants (0.94 and 0.92, respectively) indicated that over time, youth differed as much from themselves as from each other. Participants averaged 2.4 different risk levels; only 13% were consistent over time. Standard cross-sectional risk assessments and between-youth comparisons overlook important information, with critical implications for risk reduction intervention. Further research is needed.