Within the framework of a nationwide project, sponsored by a number of Spanish youth services agencies and the juvenile justice delivery system, 100 juveniles, chosen at random among the inmates of a Spanish correctional institution, were given thorough medical examinations and routine tests. Medical histories of the juveniles' nuclear and extended families, and data on the subjects' own biotypologies, cardiocirculatory and respiratory systems; digestive and intestinal tracts, locomotor apparatus, nervous systems, kidneys, sexual organs, skin, eyes, nose, ears, blood groups, and Rh factors were examined and analyzed. All attempts to trace the juveniles' deviant behavior to biological influences and physical pathologies failed. The subjects of this study showed few, if any, differences compared with a control group. None of the juveniles examined appeared to suffer from any venereal diseases, although by their own admission the majority of the boys had become sexually active before age 16. The vision problems found in some of the subjects were not corrected by appropriate eyeglasses, a possible reason for the migraines of which a number of the juveniles complained. One problem area was the digestive tract. The reasons for these digestive disorders, including frequent vomiting, were alcohol abuse and irregular eating habits. The causes of juvenile delinquency must, therefore, be sought in socioeconomic and environmental factors, lack of educational opportunities, and other problems unrelated to physical pathologies, although improved health habits might brighten the young offenders' outlook on life and increase their chances for social integration. Spanish and English annotations are appended. --in Spanish.