<p>Animal studies have reported associations of early maternal separation with altered μ-opioid receptor function but data on humans are scarce. We now investigated whether childhood family environment is related to μ-opioid receptor availability in the human brain in adulthood. Healthy participants (<i>n</i> = 37–39 in the analyses) were recruited from the prospective population-based Young Finns Study (YFS) that started in 1980. Childhood family environment was evaluated in 1980, including scores for stress-prone life events, disadvantageous emotional family atmosphere, and adverse socioeconomic environment. We used positron emission tomography (PET) with radioligand [¹¹C]carfentanil to measure μ–opioid receptor availability in adulthood. Age- and sex-adjusted analyses showed that exposure to stress-prone life events in childhood was related to lower μ-opioid receptor binding in the orbitofrontal cortex, hippocampus, putamen, amygdala, insula, thalamus, anterior cingulate cortex, and dorsal caudate in adulthood (when compared to participants not exposed to stress-prone life events). Unfavorable socioeconomic family environment or disadvantageous emotional family atmosphere was not associated with μ-opioid receptor availability in adulthood. In conclusion, exposure to environmental instability (i.e., to stress-prone life events below traumatic threshold) during early development is associated with dysregulation of the u-opioid receptor transmission in adulthood. The findings increase understanding of the neurobiological mechanisms involved in the associations between childhood adversities and adulthood mental disorders.</p>