The prevalence of personality disorders in penal population was studied with two instruments, the classic MMPI and the recently published IPDE, authorized by the WHO for the diagnosis of personality disorders in both, DSM-IV and ICD-10 versions. A sample of 56 prisoners from a Spanish prison was studied, mean age 22, all male, 98% of them had never requested psychiatric help. The crimes most frequently committed were related to drug traffic and drug abuse (thefts, robberies, crimes against public health). There were also cases of homicide, homicide attempt, rape and kidnapping. 91% of the studied sample presented one or more personality disorders, being the most frequent: Antisocial (79%), Paranoid (52%) and Borderline (41%). The MMPI scales most frequently obtained were: Psychopathic deviation (59%), Paranoia (46%) and Schizophrenia (41%). There was a good clinical correlation between the IPDE and the MMPI results.