Recruiting participants for large prevention trials is time consuming and costly. In order to test various recruitment techniques, we conducted two studies of response rates to recruitment mailings for the Women's Health Trial. The potential participants, 50- to 79-year-old women, were requested to return an enclosed postcard to learn more about the trial. In the first study, we sent at random either a short or a long message to a group of University of Miami personnel (N = 862) and a Dade County cohort (N = 2964). More university women responded to the short message than to the long message (22.4% vs 16.4%, p = 0.024). Similarly, more of the Dade County cohort replied to the short message than to the long one (12.1% vs 9.6%, p = 0.027). The long message listed details of the intervention (e.g., modifying recipes) that some women may have used to decide they were not interested in participating. In the second study, we examined response rates to two different ways of addressing the mailing, i.e., handwritten envelopes and machine-printed labels; we also evaluated three methods for delivering the short message: (1) formal invitation, (2) business letter with an inside name and address of the recipient, and (3) business letter without the recipient's name and address. Response rates were similar between the methods of addressing envelopes and among the three vehicles for the message, suggesting that the least costly method of mailing should be used.