OBJECTIVE: To examine the impact of postal invitations and user fee on influenza vaccination rates. DESIGN: A controlled randomized trial in 13 general practices. One third of the participating patients received postal invitations to influenza vaccination free of charge. Another third received postal invitations to influenza vaccination on paying the usual fee (US$ 40-60). The last third served as a control group, being vaccinated at their own request and paying the usual fee. SETTING: General practice in the Counties of Funen and Vejle, Denmark. PATIENTS: Five hundred and eighty-five patients aged 65 years or older, recognized by their general practitioner (GP) as being in the risk group for whom influenza vaccination is recommended. MAIN OUTCOME MEASURES: Influenza vaccination rates. RESULTS: In the control group 25% (19-31%, 95% confidence interval) of the patients were vaccinated, compared with 49% (42-56%) in the group who received a postal reminder and paid the usual fee, and 72% (65-78%) in the group invited to be vaccinated free of charge. CONCLUSION: It is suggested that GPs send postal invitations to their elderly patients in the risk groups urgently recommending influenza vaccination. Attention should also be given to offering free influenza vaccination to elderly patients who have recognized indications for vaccination.