Background: Contrasting views exist over the association of intellectual disability (ID) and criminal offending. This nationwide study attempts to shed further light to expand understanding to substantiate the relation between socio-demographic characteristics, psychiatric co-morbidity and criminal behaviour among the Finnish forensic population with ID. Method: We reviewed all forensic psychiatric examination reports of individuals with ID who underwent a pre-trial forensic psychiatric evaluation in Finland during an 11-year period (1996-2006). Results: One-third of the offenders had been regularly and sufficiently treated as outpatients. Half of the offenders had previous criminality, and the single most common crime was arson. Almost half of the offenders were diagnosed with alcohol abuse/dependence and two-thirds with any substance abuse/dependence. Furthermore, almost half were intoxicated during the index crime. Antisocial personality disorder was diagnosed in 25% of the offenders. Almost half of the offenders were placed in involuntary special care for the ID, which lasted ca 2 years. Among the last-mentioned, two-thirds of the nursing care plans lacked recommended structure. Conclusions: The offenders with so-called triple diagnosis--substance abuse, mental illness and ID--form a small subgroup of criminal offenders with complex needs. The results of the present study underline the importance of close, long-term cooperation among specialists in the field of ID, addiction service, mental health services and forensic psychiatry.