Little research has investigated interventions to improve the delivery of counselling in health care settings. We determined the impact of training and supervision delivered as part of the Options: Western Cape project on lay antiretroviral adherence counsellors' practice. Four NGOs employing 39 adherence counsellors in the Western Cape were randomly allocated to receive 53 h of training and supervision in Options for Health, an intervention based on the approach of Motivational Interviewing. Five NGOs employing 52 adherence counsellors were randomly allocated to the standard care control condition. Counselling observations were analysed for 23 intervention and 32 control counsellors. Intervention counsellors' practice was more consistent with a client-centred approach than control counsellors', and significantly more intervention counsellors engaged in problem-solving barriers to adherence (91 vs. 41 %). The Options: Western Cape training and supervision package enabled lay counsellors to deliver counselling for behaviour change in a manner consistent with evidence-based approaches.