OBJECTIVES: To examine the effects of U.S. federal parity legislation on substance use disorder treatment in the first year after passage. METHODS: We used insurance claims data from Aetna Inc. health plans in ten states with state-level parity laws for substance use disorder treatment.  We used a difference-in-differences study design to compare changes in outcomes among self-insured health plan enrollees subject to federal parity in the years before and after implementation of federal parity (2009-2010) with changes in outcomes among a comparison group of fully insured health plan enrollees previously covered by state substance use disorder parity laws (N=298,339).  Outcomes included: shares of enrollees using any substance use disorder treatment; annual total spending on substance use disorder treatment; out-of-pocket spending on substance use disorder treatment and three substance use disorder performance measures: identification, treatment initiation and treatment engagement. RESULTS: In the first year of implementation, federal parity did not lead to changes in the proportion of enrollees using substance use disorder treatment.  Total spending on substance use disorder treatment per enrollee increased more for self-insured enrollees subject to federal parity than for fully-insured enrollees ($16.11 vs. $6.12, difference $9.99, 95% CI: $2.54 to $18.21).  Federal parity was not associated with any statistically significant changes in identification, treatment initiation or treatment engagement. CONCLUSIONS: Inclusion of substance use disorder services in the federal parity law did not result in substantial increases in health plan spending.  It will be critical to evaluate the effects of federal parity in more recent data, after regulations affecting the management of care (e.g., utilization review, network access) took effect.