UNUSUALLY LONG Y-CHROMOSOMES HAVE BEEN FOUND IN MEN OF BOTH HIGH AND LOW INTELLIGENCE, AND THIS ANOMALY HAS BEEN THOUGHT TO BE MERELY A NORMAL VARIANT WITHOUT ANY ADVERSE EFFECT. A COMPARATIVE STUDY OF 84 CANADIAN CRIMINALS AND 38 MEDICAL PERSONNEL AS CONTROLS USED THE PERIPHERAL BLOOD MICROMETHOD AND FLUORESCENT PHOTOGRAPHY TO DETERMINE IF AN ATYPICAL-Y-CHROMOSOME PATTERN OCCURRED MORE OFTEN IN CRIMINAL THAN IN NONCRIMINAL MEN. MEASUREMENTS WERE TAKEN WITH PRINTS OF THE FLUORESCENCE PICTURES TO DETERMINE IF THE AVERAGE LENGTH OF THE Y-CHROMOSOMES WAS THE SAME IN BOTH GROUPS. NO ATYPICAL Y-CHROMOSOMES WERE FOUND IN EITHER GROUP, NOR WERE ANY OF THE SUBJECTS CARRIERS OF THE XYY TRAIT. THE LENGTH OF THE Y-CHROMOSOME WAS GREATER IN THE PRISONERS THAN IN THE CONTROLS. THE DIFFERENCE IN AVERAGE LENGTH WAS SIGNIFICANT (P LESS THAN 0.025). THE RESULTS DO NOT RESOLVE THE QUESTION OF WHETHER THE LONG Y CARRIERS ARE INHERENTLY MORE PRONE TO CRIMINAL BEHAVIOR, AND FURTHER RESEARCH IS NECESSARY BEFORE A CONCLUSION CAN BE MADE. TABULAR DATA AND REFERENCES ARE PROVIDED. (TWK)