Maternal filicide may be considered the result of significant interactions between increased individual vulnerability and overwhelming environmental stress. The present study examined whether the biological vulnerability to stress and psychotic depression in criminally insane filicidal women was associated with an imbalance of stress-related hormones. Early-morning plasma levels of hormones associated with depression and chronic stress, i.e., thyroid hormones, Cortisol and Adrenocorticotropic hormone (ACTH), were measured in 10 filicidal inpatients recovered in a high-security psychiatric hospital for the criminally insane and 10 matched psychiatric, non-filicidal, criminal mothers with comparable traumatic/abuse records. Filicidal mothers had higher than normative Cortisol levels and significantly higher ACTH levels than both the normative values and plasma levels of non-filicidal women. Levels of thyroid hormones fell within normal ranges, without between-groups differences. In addition, while psychiatric controls had the expected Cortisol-ACTH positive correlation, mothers who killed their children revealed no relationship between the two hormones. HPA in the group of filicide perpetrators was altered despite they had received antidepressant pharmacological treatment. The observed imbalance of hypothalamic-pituitary-adrenal (HPA) axis indicates a possible filicides' reduced sensitivity of the adrenal glands to ACTH, probably due to the pre-hospitalization long-term affective stress which preceded child homicide. The results reveal the existence of large psycho-biological stress-sensitivity in filicides, and careful post-discharge psychiatric follow-up of such women is recommended.