In a follow-up study, only 64% of 126,628 US radiologic technologists completed a questionnaire during 1994–1997 after two mailings. The authors conducted a randomized trial of financial incentives and delivery methods to identify the least costly approach for increasing overall participation. They randomly selected nine samples of 300 nonresponders each to receive combinations of no, $1.00, $2.00, and $5.00 cash or check incentives delivered by first-class mail or Federal Express. Federal Express delivery did not achieve greater participation than first-class mail (23.2% vs. 23.7%). In analyses pooled across delivery methods, the response was significantly greater for the $2.00 bill (28.9%, 95% confidence interval (CI): 25.2, 32.7; p < 0.0001), $5.00 check (27.5%, 95% CI: 22.5, 33.0; p = 0.0001), $1.00 bill (24.6%, 95% CI: 21.2, 28.3; p = 0.0007), and $2.00 check (21.8%, 95% CI: 18.5, 25.3; p = 0.02) compared with no incentive (16.6%, 95% CI: 13.7, 19.9). The response increased significantly with increasing incentive amounts from $0.00 to $2.00 cash (p trend < 0.0001). The $2.00 bill achieved a 30% greater response than did a $2.00 check (p = 0.005). For incentives sent by first-class mail, the $5.00 check yielded 30% greater participation than did the $2.00 check (p = 0.07). A $1.00 bill, chosen instead of the $2.00 bill because of substantially lower overall cost and sent by first-class mail to the remaining 42,717 nonresponders, increased response from 64% to 72%.