There is a growing concern that food or cash transfer programs may contribute to overweight and obesity in adults. We studied the impact of Mexico's Programa de Apoyo Alimentario (PAL), which provided very poor rural households with cash or in-kind transfers, on women's body weight. A random sample of 208 rural communities in southern Mexico was randomly assigned to 1 of 4 groups: food basket with or without health and nutrition education, cash with education, or control. The impact on women's weight was estimated in a cohort of 3010 women using a difference-in-difference model. We compared the impact between the food basket and cash groups and evaluated whether the impact was modified by women's BMI status at baseline. With respect to the control group, the program increased women's weight in the food basket (550 ± 210 g; P = 0.004) and the cash group (420 ± 230 g; P = 0.032); this was equivalent to 70 and 53% increases in weight gain, respectively, over that observed in the control group in a 23-mo time period. The greatest impact was found in already obese women: 980 ± 290 g in the food basket group (P = 0.001) and 670 ± 320 g in the cash group (P = 0.019). Impact was marginally significant in women with a preprogram BMI between 25 and 30 kg/m2: 490 ± 310 g (P = 0.055) and 540 ± 360 g (P = 0.067), respectively. No program impact was found in women with a BMI <25 kg/m2. Providing households with a considerable amount of unrestricted resources led to excess weight gain in an already overweight population. Research is needed to develop cost-effective behavior change communication strategies to complement cash and in-kind transfer programs such as PAL and to help beneficiaries choose healthy diets that improve the nutritional status of all family members.