This study examined neighborhood built environment characteristics (fast-food restaurant density, walkability) and individual eating-out and physical activity behaviors in relation to 1-year change in body weight among adults 50-75 years of age at baseline. The authors surveyed 1,145 residents recruited from 120 neighborhoods in Portland, Oregon. During the 1-year follow-up (2006-2007 to 2007-2008), mean weight increased by 1.72 kg (standard deviation, 4.3) and mean waist circumference increased by 1.76 cm (standard deviation, 5.6). Multilevel analyses revealed that neighborhoods with a high density of fast-food outlets were associated with increases of 1.40 kg in weight (P<0.05) and 2.04 cm in waist circumference (P<0.05) among residents who visited fast-food restaurants frequently. In contrast, high-walkability neighborhoods were associated with decreases of 1.2 kg in weight (P<0.05) and 1.57 cm in waist circumference (P<0.05) among residents who increased their levels of vigorous physical activity during the 1-year assessment period. Findings point to the negative influences of the availability of neighborhood fast-food outlets and individual unhealthy eating behaviors that jointly affect weight gain; however, better neighborhood walkability and increased levels of physical activity are likely to be associated with maintaining a healthy weight over time.