In recent years, increasing evidence of sexual abuse of children has created a new field of medicolegal inquiry requiring a new expertise. In psychoanalysis, one of our specific tasks is to address the issues of diagnosis and of the pathogenic effect of sexual abuse as it is ultimately reflected in the intrapsychic dynamics of the developing ego. My hypothesis is that it may be possible to determine early in the treatment whether sexual abuse has taken place. The case of a two-and-three-quarters-year-old girl, suspected of having been molested by her father, is presented in depth. The evolution of material confirming the diagnosis is presented by way of documenting evidence of sexual abuse. Issues elaborating the difficulty in making the diagnosis are also explored. Finally, a synthesis of diagnostic considerations useful for arriving at a diagnosis of sexual abuse is presented in the hope that these factors will serve as diagnostic guidelines.