Objective: To examine the impact on general practitioners' workload of adding nurse practitioners to the general practice team. Design: Randomised controlled trial with measurements before and after the introduction of nurse practitioners. Setting: 34 general practices in a southern region of the Netherlands. Participants: 48 general practitioners. Intervention: Five nurses were randomly allocated to general practitioners to undertake specific elements of care according to agreed guidelines. The control group received no nurse. Main outcome measures: Objective workload, derived from 28 day diaries, included the number of contacts per day for each of three conditions (chronic obstructive pulmonary disease or asthma, dementia, cancer), by type of consultation (in practice, telephone, home visit), and by time of day (surgery hours, out of hours). Subjective workload was measured by using a validated questionnaire. Outcomes were measured six months before and 18 months after the intervention. Results: The number of contacts during surgery hours increased in the intervention group compared with the control group (P <0.06), particularly for patients with chronic obstructive pulmonary disease or asthma (P <0.01). The number of consultations out of hours declined slightly in the intervention group compared with the control group, but this difference did not reach significance. No significant changes became apparent in subjective workload. Conclusion: Adding nurse practitioners to general practice teams did not reduce the workload of general practitioners, at least in the short term. This implies that nurse practitioners are used as supplements, rather than substitutes, for care given by general practitioners.