An example of folie à deux with an uncommon clinical picture is presented. It concerns two spinster sisters, with no past psychiatric history, who became simultaneously manic while on holiday. The “Principal” was clearly deluded; the “Associate” shared these abnormal beliefs. After recovery and eighteen months later, one became depressed and the other shared her suicidal ideas, but this time the roles of the “Principal” and “Associate” were reversed. The shortcomings of the ICD10 and DSM-IIIR in classifying this syndrome are highlighted.