According to several studies, the prevalence of drug use (1) among prisoners is manyfold compared to general population. However, comparisons across studies are hampered by the use of a variety of mutually noncomparable methodologies. We report substance use among Finnish prisoners using three methods and analyze these differences. The material consisted of 610 Finnish prisoners in 2006 and represents all Finnish prisoners. The subjects participated in a comprehensive field study including a standardized psychiatric interview (SCID-I). Alcohol abuse/dependence was diagnosed in 68% (SCID-I) and 72% (ICD-10) among men and 70% (both SCID-I and ICD-10) among women. Drug abuse/dependence was diagnosed in 62% (SCID-I) and 69% (ICD-10) among men and 64% (SCID-I) and 70% (ICD-10) among women prisoners. Interview data revealed that the majority had at least tried most substances. Both alcohol and drug abuse/dependence were vastly more common among Finnish prisoners than reported elsewhere. The DSM-IV-based SCID-I produced slightly lower prevalence estimates than an ICD-10 clinical examination, but overall SCID/ICD agreement was very good. It seems that physicians use information other than that captured by standardized structured clinical interview when placing a diagnosis. Nonclinical interview-based prevalence figures may overestimate harmful use of drugs unless known risk patterns of use (e.g., intravenous use) are specifically addressed.