We describe three siblings with familial infantile myasthenia gravis (FIMG). Contrary to previous reports suggesting a benign course of FIMG beyond the infantile period, our two patients had severe respiratory exacerbations in early adult life. Thymectomy was performed in the two more affected siblings, with only brief beneficial effect in one and no improvement in the other. Steroid therapy, employed in two patients who underwent thymectomy, resulted in no long-term improvement. The third patient has not required therapy. Recognition of FIMG has important therapeutic implications, because this disease shows rather poor response to thymectomy and steroid therapy.