Purpose: In this study, we investigated the effects and handling of an intervention to increase compliance with age limits regarding alcohol sales. The intervention tested in this field experiment was a feedback letter sent to alcohol outlets about their individual compliance results based on a mystery shopping study. Method: We measured compliance in 146 alcohol outlets (cafeterias, supermarkets, bars, liquor stores and youth centres) in one region in the Netherlands with 15-year-old mystery shoppers. About half (n = 72) of the outlets received the intervention letter (the experimental group). After this intervention, we measured compliance again (n = 138). Then we sent the same letter to the control group and interviewed all the outlets regarding their handling of the intervention (n = 106). Results: After the experimental group received the letter, compliance increased significantly (from 18.1% to 32.4%). In the control group, compliance did not change. Of the outlets interviewed, 81% stated that they had received the letter, and the action most commonly taken was to bring the letter to the attention of their staff. Conclusions: Positive feedback letters are more often copied and shared integrally with personnel, compared with negative letters. Compliance with respect to underage alcohol sales can be improved, although compliance levels remain low in the Netherlands.