Background. Medical events are often reported as triggers for weight loss, but it is unknown whether medical triggers result in better short- and long-term weight control. Methods. The relationship between medical triggers and weight loss was examined in the National Weight Control Registry (NWCR), a database of individuals who have lost ≥30 lbs and kept it off for ≥1 year. Recall of weight loss triggers may become difficult over time, thus participants were limited to those reporting weight loss <5 years ago. Three groups were examined: (1) participants with medical triggers (N = 207), (2) participants with nonmedical triggers (N = 539), and (3) participants with no trigger (N = 171). Results. Participants with medical triggers were older than those with nonmedical triggers or no trigger (50.5 ± 11.7, 44.9 ± 11.8, 46.7 ± 13.3 years; P = 0.0001), had a higher initial BMI at entry into the NWCR (26.1 ± 5.0, 25.0 ± 4.3, 24.8 ± 4.4 kg/m2; P = 0.004), and were more likely to be male (37.1%, 18%, 17.2%; P = 0.0001). Participants with medical triggers reported greater initial weight loss than those with nonmedical triggers or no trigger (36.5 ± 25.0, 31.8 ± 16.6, 31.8 ± 17.1 kg; P = 0.01). Participants with medical triggers also gained less weight over 2 years of follow-up than those with nonmedical triggers or no trigger (P = 0.003). Conclusions. Medical triggers may produce a teachable moment for weight control, resulting in better initial weight loss and long-term maintenance. © 2004 The Institute For Cancer Prevention and Elsevier Inc. All rights reserved.