Evidence about the mental health consequences of unaffordable housing is limited. The authors investigated whether people whose housing costs were more than 30% of their household income experienced a deterioration in their mental health (using the Short Form 36 Mental Component Summary), over and above other forms of financial stress. They hypothesized that associations would be limited to lower income households as high housing costs would reduce their capacity to purchase other essential nonhousing needs (e.g., food). Using fixed-effects longitudinal regression, the authors analyzed 38,610 responses of 10,047 individuals aged 25-64 years who participated in the Household, Income, and Labour Dynamics in Australia (HILDA) Survey (2001-2007). Respondents included those who remained in affordable housing over 2 consecutive waves (reference group) or had moved from affordable to unaffordable housing over 2 waves (comparison group). For individuals living in low-to-moderate income households, entering unaffordable housing was associated with a small decrease in their mental health score independent of changes in equivalized household income or having moved house (mean change = -1.19, 95% confidence interval: -1.97, -0.41). The authors did not find evidence to support an association for higher income households. They found that entering unaffordable housing is detrimental to the mental health of individuals residing in low-to-moderate income households.