Numerous studies have demonstrated that low birth weight (LBW) is associated with the development of medical conditions, such as hypertension and diabetes, and psychiatric disorders, such as depression. One possible mechanism through which LBW might increase risk for both medical and psychiatric disorders is by altering the biologic systems (such as the hypothalamic-pituitary-adrenal [HPA] axis function) that govern emotion regulation and physical reactivity. In this study, we conducted secondary data analyses in a longitudinal study originally designed to understand the intergenerational transmission of major depressive disorder (MDD). We examined the risk for both medical and psychiatric illnesses known to be influenced by HPA axis dysregulation in the context of parental depression. The study had 2 primary objectives: (1) to examine whether LBW increases the risk of selected adult illness that may be influenced by the HPA axis and (2) to examine whether the increased risk of illness varies by parental depression status. We conducted longitudinal assessments of 244 offspring of depressed and nondepressed parents for more than 20 years. Psychopathology and medical illness were assessed by direct interview conducted by clinicians blind to risk status and previous diagnosis. We examined the effect of BW in 3 categories: less than 2.5 kg (LBW), 2.5-3.5 kg, and more than 3.5 kg (reference group). Offspring with LBW had a significantly increased risk of MDD, anxiety disorders, phobia, suicidal ideation, impaired functioning, allergies, and hypertension compared to those with BW exceeding 3.5 kg. The association between LBW and depression was stronger among children of depressed parents than among children of nondepressed parents, with an interaction term (BW and parental depression status) significant for MDD (P = .05), suggesting that parental depression may augment the impact of LBW on offspring depression: