question: OBJECTIVES: Household chaos, including disorder, noise, and crowding within the home, is a risk factor for poor mental and physical health. Household chaos may act upon maternal behaviors of physical activity and sleep, potentially via higher stress. The purpose of this study is to evaluate the relationships among household chaos, maternal stress, and maternal physical activity and sleep, and identify barriers to home organization during the COVID-19 outbreak. METHODS: A cross-sectional study using an online survey of 1721 mothers of preschoolers (ages = 3.0-5.9 years) in the United States was conducted in May 2020 during COVID-19 stay-at-home orders and early reopening. Mothers reported demographic characteristics, household chaos, stress, physical activity and sleep, and barriers to home organization during the outbreak. Mediation models were conducted among household chaos, stress, and physical activity and sleep with adjustment for covariates. RESULTS: About half of mothers were middle income (48.2%), employed full-time prior to the outbreak (59.1%), and met the physical activity (47.7%), and sleep guideline (49.7%, 7-9 h/day). Household chaos and stress were both negatively related to physical activity and sleep. For every 1 unit increase in mother's current stress, mothers were 11% (95% confidence interval = 6% to 16%) less likely to meet the physical activity guideline and 19% (95% confidence interval = 14% to 23%) less likely to meet the sleep guideline. Household chaos was positively related to stress. Stress partially mediated the relationship between household chaos and physical activity and sleep. Virus concerns, occupational changes (i.e. teleworking), and lack of childcare were barriers to home organization. CONCLUSION: During the COVID-19 outbreak, many mothers had poor sleep and physical activity, which was associated with household chaos and stress. Opportunities to promote order at the individual, household, and community level may result in beneficial mental and physical health in mothers of young children during the COVID-19 outbreak and beyond.