Sex differences are examined for the period during which addiction develops after initial narcotics use. About 25% of the 546 heroin addicts studied became addicted within 1 month after initial use. More women than men fell into this category, and, on the average, women took less time to become addicted. For the majority of those studied (whose addiction occurred 1 month or longer after initial use), patterns of narcotic use during this phase of the addiction career were not significantly different by sex within Anglo or Chicano groups. Differences between female and male addicts in interpersonal relationships, other substance use, drug dealing, legal employment, and criminal behaviors parallel traditional sex role expectations.