This article reports the results of an experiment in obtaining physician response to a mailed questionnaire. Each physician was eligible for a payment of $20. A randomly selected half received the payment with their initial questionnaire and cover letter; half were told they would receive their payment after they completed and returned the questionnaire. The same mail and telephone followup procedures were used for both groups. Overall, prepayment had significant positive effects on response rates. This paper examines these effects in terms of response rates for various specialties, field efficiencies, cost, and representativeness of the sample.