Weekly, physical activity should be a primary component in long term, healthy lifestyles to enhance aerobic capacity and potentially decrease risk of cardiovascular disease (CVD). Start The Cycle (STC) is a non-profit, community-based program dedicated to building confidence and self-esteem in at-risk youth (e.g., helping them achieve healthy, life goals) through weekly mountain bike rides and adult mentorship. PURPOSE: To quantify physiological change and probability of CVD in at-risk youth during a 16-wk, mentored mountain biking program. METHODS: Participants included (mean ± SD) new members (NM, n = 15, age = 13.6 ± 1.8 yrs), returning members (RM, n = 15, age = 15.9 ± 2.3 yrs), and combined (NM + RM) members (CM, n = 30, age = 14.7 ± 2.4 yrs). Free mountain bikes were provided to participants by STC with a promise of ownership if the full program was completed. The program met 16-wks, 1 x week, and 2-hrs·day-1 starting late spring and into late summer. Indoor physical conditioning and bike maintenance + skills classes were implemented the initial 4-wks with mentored, group rides occurring the last 12-wks. Maximal oxygen uptake (i.e., via the progressive aerobic cardiovascular endurance run or PACER test) and CVD risk (via a prediction equation from prior, unpublished research) were assessed pre- and post-intervention following the indoor training sessions (i.e., after 4-wks) and immediately prior to a final, 28-mile organized bike race. Data were analyzed using paired t-tests between pre- and post-intervention within NM, RM and CM groups with significance set at p < 0.05. Effect size is reported as Cohn’s d with d = .2, .5, and .8 = small, medium, and large effect sizes, respectively. RESULTS: Significance from pre- to post-test (mean ± SD), respectively, is as follows for VO2max in ml·kg-1·min-1 (NM, 37.1 ± 2.5 vs 41.8 ± 5.4, p = 0.004, d = 0.98; RM, 40.7 ± 6.9 vs 46.2 ± 10.9, p = 0.037, d = 0.19; and CM, 38.7 ± 5.1 vs 43.8 ± 8.3, p = 0.000, d = -0.04) and CVD risk with lower score = decreased risk (NM, 5.2 ± 2.8 vs 3.8 ± 2.8, p = 0.018, d = -0.38; RM, 6.2 ± 6.2 vs 3.9 ± 8.0, p = 0.027, d = 0.19; and CM, 5.7 ± 4.7 vs 3.8 ± 5.8, p = 0.001, d = -0.07). Body mass index (BMI) did not change across any group. CONCLUSION: A community-based, adult-mentored, youth mountain bike program is a practical means to improve aerobic capacity and reduce CVD risk in at-risk minors.