Secondary analysis of a randomized trial.To examine the influence of experience and specialty certification on outcomes for patients with low back pain receiving a standardized manipulation or stabilization exercise intervention program.Little research has examined the impact of therapist-related factors on the outcomes of clinical care for patients with low back pain. It is assumed that therapists with more clinical experience or specialty certification will achieve better clinical outcomes; however, few studies have examined this hypothesis.One hundred thirty-one participants in a randomized trial were included (70 randomized to receive manipulation, 61 stabilization). All subjects completed an Oswestry Disability Questionnaire at baseline, and after 1 and 4 weeks of treatment. Therapists were categorized based on total years of experience, years of experience with manual therapy, and specialty certification status. Two-way repeated-measures analyses of covariance were performed within each intervention group to examine the effects of the therapist characteristics on outcomes. Hierarchical linear regression models were used to examine the relative effects of therapist characteristics and intervention on clinical outcomes.Thirteen therapists participated (average 6.0 years of experience [standard deviation, 4.01, 4 (30.8%) with specialty certification). A significant interaction between time and specialty certification status (P = .04) was detected for subjects receiving the manipulation intervention. No significant interactions were detected in the stabilization group. The regression models found that the intervention group significantly contributed to explaining clinical outcomes, but that therapist characteristics did not.With the standardized protocol utilized in this study, it appears that the therapist-related factors of increased experience and specialty certification status do not result in an improvement in patients' disability associated with low back pain.