Adult hexosaminidase A deficiency is a form of GM2 gangliosidosis with autosomal recessive inheritance. Only 35 cases (mostly among Ashkenazic Jews) have been reported worldwide. Symptoms include, in a third of the cases, psychosis. A 27–year-old sufferer with no prior psychiatric history, developed a post-partum psychosis, with affective and hebephrenic components, 3 days following her first delivery. She responded to lithium within 10 days of initiating treatment; the full episode lasted 1 month. We conclude that lithium is the preferred treatment for psychosis in such adult patients, especially in light of possible long-term neurological deterioration caused by phenothiazines. Ashkenazic Jews with atypical neurological syndromes presenting with psychosis should be tested for hexosaminadase A deficiency.