THE CHROMOSOME ANALYSIS WAS MADE ON LEUCOCYTES OBTAINED FOR 48 HOURS AND THE Y AND F CHROMOSOMES WERE MEASURED IN FIVE WELL-SPREAD METAPHASES DURING WHICH THE Y COULD BE DISTINGUISHED EASILY. THE F CHROMOSOMES WERE MEASURED DIAGONALLY FROM THE END OF ONE CHROMATID TO THE END OF THE OPPOSITE CHROMATID, AND THE Y WAS MEASURED FROM THE END OF THE SHORT ARMS TO THE END OF THE LONG ARMS. THE DISTRIBUTION WAS NORMAL FOR BOTH GROUPS, BUT THE MEAN LENGTH OF THE Y CHROMOSOME WAS SIGNIFICANTLY HIGHER IN THE CRIMINAL MALES, COMPARED WITH CONTROLS. THE FINDINGS INDICATE THAT THERE IS A CORRELATION BETWEEN THE SIZE OF THE Y AND THE RISK OF CRIMINALITY, WITH AN INCREASED RISK OF CRIMINALITY AMONG MALES WITH A COMPARATIVELY LARGE Y CHROMOSOME. PREVIOUS STUDIES HAVE SHOWN THAT THE VARIATION IN THE LENGTH OF THE Y IS DUE TO VARIATION IN THE FLUORESCENT PART OF THE LONG ARMS; THESE FINDINGS REVEAL THAT THIS VARIATION IN LENGTH OF THE FLUORESCENT PART OF THE Y IS DUE TO VARIATION IN CHROMOSOME MATERIAL AND NOT JUST TO A DIFFERENCE IN SPIRALIZATION. TABULAR DATA AND REFERENCES ARE PROVIDED. (DAG)