Abstract Background In theory, Motivational Interviewing (MI) fidelity should be associated with client outcomes. Nevertheless, this fidelity-effectiveness association is rarely investigated. This study evaluated the extent to which Telephone Motivational Interviewing (TMI) fidelity is associated with change in self-reported physical activity (PA), fruit intake, and vegetable consumption. Method Adults in primary care (45–70 years) participated in a study that compared the effect of tailor print communication, telephone motivational interviewing (TMI), and a combination of the two on PA, fruit intake, and vegetable consumption. MI fidelity was assessed using the behavioral coding method “Motivational Interviewing Treatment Integrity Code (MITI)” in 409 randomly selected audio-recorded sessions, representing 232 participants of the TMI group. The associations between MI fidelity scores and the behavioral changes from baseline to 47-week follow-up were examined using backward multiple linear regression analyses (adjusted for covariates). Results A significant and positive association between the percentage of MI adherent responses and improvements in PA and fruit consumption was found with respectively a small and medium effect size. The global rating “Spirit” (which resembles an all-at-once appraisal of the interviewer’s MI competence) was significantly, but inversely associated with progress in vegetable intake with a medium effect size. Conclusion The finding that relatively lower MI competency was associated with higher vegetable consumption went against our expectations. Findings suggest that practicing MI-consistent skills was beneficial in promoting PA and fruit consumption, but moderated vegetable intake. This study contributes to the scientific confidence that TMI enables change in PA and fruit intake.