- 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? 5k g 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.