In the following, a case of mother-adult son incest is described and explained from a psychoanalytical viewpoint. Two theories are put forward: (a) Mother-son incest may occur as a defence against psychosis, and (b) the incest represents an unconscious search for triangulation, a process in which external authorities (such as, for example, a court of law) may function as surrogates for persons who have been missed in the pre-oedipal past. It is therefore possible to understand mother-son incest symbolically as an indicator of pre-oedipal needs of the son and of the mother's longing for the absent partner. The incest is, however, not only a cry for help; it is also to be regarded as an attempt to solve the problem for both people involved. Looked at in this way, new ways of understanding and new possibilities for therapy emerge.