Background: Poor adherence to oral bisphosphonates is a challenge to treatment and prevention of osteoporosis. The Veterans Health Administration (VA) operates the largest integrated health care system in the United States and offers certain advantages to possibly improve medication adherence. We aimed to determine adherence to weekly alendronate for osteoporosis in Veterans, and investigate predictors and outcomes related to adherence. Methods: A retrospective study cohort was generated from VA databases selecting Veterans who were treated with weekly alendronate. Adherence was measured by medication possession ratio (MPR) and persistence. Two groups were defined as low and high adherence based on MPR <80% or ≥80%, respectively. Regression models were used to investigate predictors of adherence and included clinically relevant covariates. Further regressions were used to investigate the impact of adherence on change in bone mineral density measured by dual energy X-ray absorptiometry and incident fracture. Results: In a cohort of 913 (female/male, 207/706) Veterans, 48% had high adherence in year 1. Distribution for gender, race, and age were similar between the 2 groups, MPR <80% or MPR ≥80%. Baseline fracture [odds ratio OR: 0.64, 95%CI: (0.41, 0.98)], alcohol abuse [0.40 (0.21, 0.74)] and tobacco use [0.44 (0.31, 0.63)] were associated with low adherence in the unadjusted analyses, but only tobacco use [0.45 (0.30, 0.67)] was associated with low adherence after adjustment. Among males, tobacco use was associated with low adherence while prostate cancer predicted high adherence in adjusted models. High adherence was associated with a 30% [hazard ratio HR: 0.70, 95% CI: (0.47, 1.03)] decreased risk of incident fracture in the whole cohort, and a 40% [0.60 (0.38, 0.95)] decrease risk in males. Conclusion: Year one adherence to weekly alendronate was a relevant determinant to long-term clinical outcomes including changes in bone mineral density and incident fracture in Veterans.