This work deals with the problem of paternal incest occurring with young daughters at puberty. Only those incestuous relations which lasted at least one year have been the subject of the present research. 1) As far as the family structure is concerned, this study has revealed the incestuous father as a weak, masochistic, passive person who is dominated by his wife. The incestuous seductions of the daughter are experienced by her as making her party to his humiliation by his wife. It is under cover of pity that he turns his erotic desires to his daughter. He either beseeches her understanding and compassion or he uses violence. He has no control over his home and above all he has no control over himself. The wife holds the reins. The father is incapable of saying no to his incestuous desires. The mothers of these adolescent girls were described as essentially rejecting, and we noted that there is a parallel between the ego-disorganization of the adolescent girl and the intensity of the mother's rejection. 2) We have found that for the duration of the incestuous relationship, these adolescent girls had not suffered any behavioural or personality disorganization which would have alarmed their environment. 3) It was at the time, and very often on the very day, of the father's departure that these girls broke down. 4) Their disorganization was characterized by a very violent, compulsive acting out, which was for the most part erotic-heterosexual behaviour. Other acting out was observed, such as suicidal attempts, running away from home, auto-mutilation and destructive behaviour. We have given the name of compulsive-masochistic reaction to this type of disorganization. Two adolescent girls eventually had a more severe reaction. They evolved a frank psychosis. 5) Our main hypothesis, which was confirmed by the analysis of the clinical material, was that the compulsive-masochistic reaction is due to a masochistic father-fixation. In other words, these young girls seek at all costs to perpetuate the incestuous liaison to which they are henceforth irremediably attached. Moreover this father-fixation protects them from a much more radical regression; namely an oral and murderous invasion by the mother. The psychosis occurs when this father-fixation is no longer possible, that is when the father is annihilated, as much in the mother's eyes as in the daughter's. (“He no longer exists for me”, Lise told us). Then the adolescent girl cannot seek any help in her struggle against the oral, devastating and totally destructive mother with whom she is at grips. The effect is therefore fatal: a collapse of the ego's structure; it is psychosis.