THE LENGTH OF THE Y-CHROMOSOME NORMALLY SHOWS CONSIDERABLE VARIATION, INCLUDING DIFFERENCES ACCORDING TO RACE. PUBLISHED INVESTIGATIONS DEALING WITH THE OCCURRENCE OF LONG Y-CHROMOSOMES AMONG MENTALLY DISORDERED MALE PATIENTS HAVE GIVEN CONFLICTING RESULTS. SEEKING TO CLARIFY THE ISSUE, INVESTIGATORS USED A PHOTOMETRIC TECHNIQUE TO MEASURE THE Y-CHROMOSOMES OF 106 MEN WHO HAD UNDERGONE PSYCHIATRIC EVALUATION AT A SWEDISH HOSPITAL. A CONTROL GROUP CONSISTED OF 49 MEDICAL STUDENTS AND PERSONNEL. NO STATISTICALLY SIGNIFICANT DIFFERENCES WERE FOUND WITH RESPECT TO Y-CHROMOSOME LENGTH IN A COMPARISON OF THE TWO GROUPS. THE ACCUMULATED EPIDEMIOLOGICAL RESEARCH TENDS TO DISPROVE ANY ASSOCIATION BETWEEN THE PRESENCE OF LONG Y-CHROMOSOMES AND CRIMINAL-PSYCHOTIC BEHAVIOR. EARLY STUDIES WHICH FOUND SUCH AN ASSOCIATION MAY HAVE FAILED TO CONSIDER THE NORMAL DIFFERENCES IN CHROMOSOME LENGTH FOUND IN DIFFERENT RACES. ISOLATED EXAMPLES MAY, HOWEVER, EXIST IN WHICH ABNORMAL BEHAVIOR IS FOUND IN INDIVIDUALS WITH ABNORMAL CHROMOSOME LENGTH. TABULAR DATA AND REFERENCES ARE PROVIDED. (TWK)