As part of a mailed health survey, we investigated the effect on the response rate of a request to explain refusal to participate. Subjects (N = 1,240) were randomized either to receive or not to receive, with the first mailing, a letter requesting an explanation of their decision not to fill out the questionnaire, if they chose that option. There was a slightly higher cumulative response during most of the study from subjects who had been sent the request, but little difference between the two study groups in the ultimate response rate [80% from the intervention group vs 83% from the control group; response rate difference = -3%; 95% confidence limits (CL) = -7%, 1%]. Of 209 individuals who were sent the request and did not return the questionnaire, only 15 (7%) sent back an explanation. A request to explain a refusal to participate in a mail survey neither jeopardized the response rate nor enhanced it.