Common myths about sex offenders are that treatment is ineffective, that the plethysmograph is a sexual lie detector, and that people can fake responses. In fact, the plethysmograph is being used in 400 programs that treat sex offenders and makes it possible to learn the sexual interests of males and females. One of its uses is in confronting the offender's denial. Another advantage rests on the knowledge that as many as 30 percent of the victims are situational rather than being the most desired person; therefore, the plethysmograph is useful in following the progress of therapy. If behavior change is taking place, the arousal to deviant stimuli should diminish and the arousal to appropriate stimuli (consenting adult) should increase. Case examples, figures, and 13 references