What predicts the effectiveness of primary health care teams? We address this question through research with 528 members of 68 primary health care teams in the UK, examining the influence of team composition and processes upon team effectiveness. Six months after measuring team composition and processes, we asked team members to rate the effectiveness of their teams in relation to teamwork, organisational efficiency, health care and patient centred care. Team size, tenure and budget holding status did not predict team effectiveness. However, team processes (objectives, participation, quality emphasis and support for innovation) accounted for 23% of the variation between teams in their effectiveness. In particular, clarity of and commitment to team objectives was key in predicting the overall effectiveness of the primary health care team. These findings are discussed in relation to the need to adopt appropriate organisational supports for teamworking in health care contexts.