Our purpose was to evaluate the interdisciplinary aspects of Project MAINSTREAM, a faculty development program that trained 39 competitively selected health professional tutors in substance abuse education. Mid-career faculty fellows (tutors) from 14 different health professions across the US dedicated 20% of their academic time for two years to Project MAINSTREAM. Teams of three fellows carried out curricular enhancement and service-learning field project requirements in mentored Interdisciplinary Faculty Learning Groups (IFLGs). Formative and summative evaluations were conducted via written questionnaires and confidential telephone interviews. The importance of interdisciplinary education was rated positively (mean of 3.57 on 1 - 5 scale). Using 18 parameters, fellows preferred interdisciplinary over single disciplinary teaching (means ranged from 3.40 - 4.86), and reported high levels of benefit from their interdisciplinary collaborations (means ranged from 3.53 - 4.56). Fellows reported that interdisciplinary educational collaborations were feasible (3.31) at their home institutions. The majority (63%) said that their trainees, colleagues, supervisors and institutions valued interdisciplinary training either "highly" or "somewhat", but 22% did not value it. The fellows identified scheduling conflicts (3.46), and lack of faculty rewards (3.46) such as pay or credit toward promotion, as two barriers that they encountered.