Objective: We examined the halo effect of a 2-year weight-loss diet trial, the Dietary Intervention Randomized Controlled Trial (DIRECT), on the weight and nutritional patterns of participants' spouses. Design: DIRECT participants in a research centre workplace were randomly assigned to one of three diets: Low-fat, Mediterranean or Low-carbohydrate. A sample of wives of the DIRECT participants, who attended support update meetings specific to their husband's diet during the first 6 months, were followed for 2 years. Setting: South Israel. Subjects: Seventy-four women (mean age 5 51 years, mean BMI 526?6 kg/m 2 ). Results: Among the wives of husbands randomised to the Low-fat, Mediterranean and Low-carbohydrate diet, self-reported weight change was respectively 21?48 kg, 22?30 kg and 24?62 kg after 6 months, and 10?39 kg, 23?00 kg and 22?30 kg after 2 years. Weight loss among wives whose husbands were in the alternative diet groups combined (Mediterranean1Low-carbohydrate) was significantly greater than among wives whose husbands were in the Low-fat group after 6 months (P 5 0?031) and 2 years (P 5 0?034). Overweight wives experienced more weight loss. The weight change of couples was significantly correlated (r 5 0?42, P , 0?001). Across all dietary groups, wives had significant improvement in their dietary patterns in all food groups according to their husbands' diets, mainly by a larger significant decrease in carbohydrate consumption in the Low-carbohydrate group (P 5 0?013 compared to Low-fat). Sixmonth weight change among the seventy-four DIRECT participants whose wives took part in the group support sessions was 25?2 kg, compared to 23?5 kg among the 248 DIRECT participants whose wives did not take part in these sessions (P 5 0?020). Conclusions: Focusing on the couple as a unit could provide a cost-effective approach to weight-loss programmes.