BACKGROUND: Low response rates are acknowledged as a potential source of bias in survey results. Response rates are a particular problem in surveys of GPs. Thus, the methods used to encourage response to mailed surveys and the influence of inducements in maximizing response rates are fundamental issues to be examined when addressing the problem of response bias. AIM: To increase the overall response rate to a national study of GPs and to explore the effects of financial and non-financial inducements on response rates. METHODS: Two mailing waves of a postal questionnaire to a 20% random sample of all GPs in England and Wales had achieved a 33% response rate. For the third mailing wave, the non-responding GPs were then divided into a control group, a group who were offered a donation to charity to complete the questionnaire and a group who were offered cash. The charity and cash groups were further subdivided into 5 pounds and 10 pounds groups to assess the effect of the size of the inducement offered. For the control group, a fourth wave was sent the offer of a 5 pounds or 10 pounds incentive. RESULTS: Response was positively affected by the offer of an inducement. Cash, however, had a more substantial effect than the offer of a donation to charity. Older GPs were less likely to participate overall, whereas male GPs were more likely to respond to a cash inducement. Doctors who had seen more patients were less likely to reply earlier and were more likely to respond to the offer of cash. CONCLUSIONS: Primary care is going through many changes, some of which have increased the workload of the GP. It may now be that, to achieve the response rates needed to validate policy-related research, the offer of inducements will become a necessary part of the research process.