Intergenerational coresidence is at a 30-year high. Studies find that economic, familial, and demographic factors shape the likelihood of this arrangement. We use NLSY79 and NLSY79YA data (2000–2014; N = 3,092) to examine how the mental health and substance use of both adult children and their mothers matter for coresidential biographies, estimating risks of moving out of and returning to their mothers’ households. Adult children who drink, smoke, or have more depressive symptoms, or whose mothers drink or smoke, are more likely to leave their mother’s household; adult children with more depressive symptoms and who smoke are more likely to return. Our findings show that children’s and mothers’ health are key determinants of coresidential patterns, suggesting that it is not just family arrangements that impact health but health that impacts family arrangements. As intergenerational coresidence increases, researchers should continue to look beyond economic, familial, and demographic determinants of coresidence to health dynamics.