Most users of illicit drugs are employed adults, with substance use rates especially high in the construction industry. In an effort to shed light on the nature and extent of drug use among construction industry workers, and to compare drug use assessment methods, substance use among construction workers, 60% of whom were apprentices, across six sites was assessed by questionnaire, urinalysis, and hair analysis. Nearly 17% of the participants reported current drug use, although drug use differed dramatically by site. Drug use rates also differed by respondent characteristics, participation rates, and assessment method. The strengths and weaknesses of each assessment method are discussed, along with the rationale for combining methods.