To compare the impact of unconditional and conditional financial incentives on response rates among Australian general practitioners invited by mail to participate in an online survey about cancer care and to investigate possible differential response bias between incentive groups.Australian general practitioners were randomly allocated to unconditional incentive (book voucher mailed with letter of invitation), conditional incentive (book voucher mailed on completion of the online survey), or control (no incentive). Nonresponders were asked to complete a small subset of questions from the online survey.Among 3,334 eligible general practitioners, significantly higher response rates were achieved in the unconditional group (167 of 1,101, 15%) compared with the conditional group (118 of 1,111, 11%) (P = 0.0014), and both were significantly higher than the control group (74 of 1,122, 7%; both P < 0.001). Although more positive opinions about cancer care were expressed by online responders compared with nonresponders, there was no evidence that the magnitude of difference varied by the incentive group. The incremental cost for each additional 1% increase above the control group response rate was substantially higher for the unconditional incentive group compared with the conditional incentive group.Both unconditional and conditional financial incentives significantly increased response with no evidence of differential response bias. Although unconditional incentives had the largest effect, the conditional approach was more cost-effective.