Guided by a review of follow-up studies concerning the Borderline Personality Disorder, it is shown that the characteristic symptoms of the Borderline Personality Disorder are connected to a certain life-phase, beginning in adolescence and ending in the fifth decennium of life. Hypotheses, explaining the lessening of symptoms and the improvement of global functioning of the patients after this life-phase, are given. Knowledge and understanding of the course of the Borderline Personality Disorder implicate several consequences for treatment. These concern the importance of suicideprevention, a possibly more broad indication for neuroleptic treatment, the favourable influence, which a `training in controlling impulses' could exert and the degree of distance, which the patients should learn to maintain in their object-relations.