This meta-analysis synthesized the findings from randomized controlled trials (RCTs) of motivational interviewing (MI) for health behavior outcomes within primary care populations. Published and unpublished RCTs were identified using databases and online listservs. Studies were synthesized by outcome subgroup and meta-regression analyses were conducted to determine potential moderators accounting for heterogeneity within samples. Mean effect sizes ranged from .07 to .47; significant effect sizes were found for the adherence subgroup of studies (p = .04) and all outcomes combined (p = .02). Professional credentials of intervention deliverer were found to significantly moderate the association between MI and effect size in substance use subgroup (p = .0005) and all outcomes combined (p = .004). Mean effect sizes were largest in outcomes related to weight loss, blood pressure, and substance use. MI appears to be useful in clinical settings and as few as 1 MI session may be effective in enhancing readiness to change and action directed towards reaching health behavior-change goals.