RATIONALE, AIMS AND OBJECTIVES: This study investigates reasons why general practices achieve nationally set milestones to different extents. It compares the beliefs, self-reported behaviours and organizational context of general practitioners (GPs) who have been successful in achieving milestones set out in the UK's National Service Framework (NSF) for Coronary Heart Disease (CHD) with those who have been less successful. METHODS: Sixteen London GPs were interviewed, eight 'high implementers' (having met five or more of six CHD NSF milestones) and eight 'low implementers' (having met one or two milestones). Practices were matched for practice size across the groups as far as possible. The interview consisted of open-ended questions, based on theoretical constructs identified as key to implementation research in a previous project. Interviews were transcribed and analysed with Interpretative Phenomenological Analysis (IPA). RESULTS: There were three main areas that differentiated high and low implementers: beliefs about evidence-based practice, control over professional practice and consequences of achieving the milestones. Low implementers: (i) expressed less belief in evidence-based guidelines as the basis of their practice; (ii) were more concerned about their lack of control over the development and implementation of the guidelines (lack of ownership), and over their own practice (lack of autonomy); and (iii) perceived more negative consequences and fewer positive consequences, both for themselves and for patient care. CONCLUSIONS: This study demonstrates the application of psychological theory in trying to understand and improve professional practice. The results suggest areas that could be targeted in developing interventions to increase guideline implementation in primary care.