THE SEXUAL INTEREST INDEX USED WAS A VOLUNTARY VIEWING TIME MEASURE, AND SEXUAL AROUSAL WAS ASSESSED BY MEASURING PENIS DIAMETER INCREASES IN MALE PATIENTS COMMITTED TO GREAT BRITAIN'S BROADMOOR HOSPITAL FOR ABNORMAL SEXUAL BEHAVIOR (USUALLY INCLUDING THE ELEMENT OF VIOLENCE). NORMAL NONPATIENTS WERE TESTED AS A CONTROL GROUP. THE MERCURY LOOP METHOD WAS USED TO MEASURE GENITAL RESPONSE, HEART RATES WERE RECORDED TO GIVE A CRUDE INDEX OF GENERAL AUTONOMIC AROUSAL CHANGES, AND PENIS SKIN BLOOD VOLUME WAS MEASURED. A SERIES OF 36 SLIDES INVOLVING HETEROSEXUAL ACTIVITIES WERE SHOWN FOR 10 SECONDS EACH, WITH THE PATIENT RATING EACH SLIDE (AS A VALIDATION TECHNIQUE). A VIDEOTAPE OF SEX FILMS WAS PRESENTED IN BLACK AND WHITE COVERING A WIDE RANGE OF SEXUAL ACTIVITIES BETWEEN INDIVIDUALS OF DIFFERENT SEXES AND AGES. THROUGHOUT THE ENTIRE PROGRAM, THE PATIENT'S VIEWING OF THE SCREEN WAS MONITORED. FINDINGS SHOWED THAT NUMEROUS PATIENTS TESTED UNDER CONDITIONS OF INDETERMINATE DETENTION MAY ADOPT A NUMBER OF DEFENSIVE STRATEGIES IN REPORTING THEIR LEVEL OF SEXUAL AROUSAL. RESULTS ALSO INDICATE THAT PATIENTS ARE IDIOSYNCRATIC IN THE EXTENT TO WHICH THEY ARE LIKELY TO SHOW SKIN BLOOD VOLUME INCREASE OR DECREASE WITH INCREASING PENIS DIAMETER. REACTIONS TO DEPICTIONS OF NORMAL ADULT HETEROSEXUAL ACTIVITY, ADULT HOMOSEXUAL BEHAVIOR, AND NUDE BOYS PRODUCED NO DIFFERENTIATION BETWEEN GROUPS OF PATIENTS AND NONPATIENTS, ACCORDING TO NONPARAMETRIC TESTS. HOWEVER, OFFENDERS AGAINST YOUNG GIRLS AND WOMEN SHOWED A SIGNIFICANTLY HIGHER LEVEL OF SEXUAL AROUSAL TO A FILM INVOLVING RAPE OF A YOUNG GIRL, AS WELL AS TO SEEING HER FULLY CLOTHED. THE ASPECT OF VIOLENCE MAY ACCOUNT FOR THEIR AROUSAL TO VIOLENT RAPE. TABLES SHOW MEASUREMENTS ACCORDING TO CATEGORIES OF SEX OFFENDERS, AND 12 REFERENCES ARE PROVIDED.