Objectives: To examine whether a weight loss program delivered to one spouse has beneficial effects on the untreated spouse and the home environment. Methods: We assessed untreated spouses of participants in three sites of Look AHEAD, a multicenter randomized controlled trial evaluating the impact of intentional weight loss on cardiovascular outcomes in overweight individuals with type 2 diabetes. Participants and spouses (n=357 pairs) were weighed and completed measures of diet and physical activity at 0 and 12 months. Spouses completed household food and exercise environment inventories. We examined differences between spouses of participants assigned to the intensive lifestyle intervention (ILI) or to the enhanced usual care (DSE; diabetes support and education). Results: Spouses of ILI participants lost -2.2±4.5 kg vs -0.2±3.3 kg in spouses of DSE participants (P<0.001). In addition, more ILI spouses lost ≥5% of their body weight than DSE spouses (26 vs 9%, P<0.001). Spouses of ILI participants also had greater reductions in reported energy intake (P=0.007) and percent of energy from fat (P=0.012) than DSE spouses. Spouse weight loss was associated with participant weight loss (P<0.001) and decreases in high-fat foods in the home (P=0.05). Conclusion: The reach of behavioral weight loss treatment can extend to a spouse, suggesting that social networks can be utilized to promote the spread of weight loss, thus creating a ripple effect. © 2008 Macmillan Publishers Limited All rights reserved.