Childhood and adult life experiences of 50 heroin addicts are compared with those of their nonaddicted brothers. The sibling pairs came from large families with a median of six children; 45 pairs came from Mexican-American families. The median age of both addicts and brothers was 39. One-third of both addicts and brothers lost a parent before age 16. The addicts showed early deviant behavior preceding heroin use in contrast to the socially conforming behavior of the brothers. Noteworthy discrepancies appeared in the retrospective explanations offered by the addicts and the brothers of conditions leading to addiction in one and abstinence in the other. The addicts attributed the addiction versus abstinence primarily to association with different peer groups; the brothers cited this difference, but also frequently cited personality differences and other differences, thus giving more complex explanations. As adults the addicts showed gross social impairment on dimensions of employment, criminal record, marital adjustment, and other life activities. The brothers, in contrast, seemed socially competent, but they nonetheless showed evidence of some impairment: 40% were arrested one or more times, and 50% had histories of alcohol abuse. Apparently the brothers did not entirely escape the adverse early environments which shaped the careers of the addicts.