The isolated appearance of Langerhans cell histiocytosis in the thymus is quite rare. We report the finding of multiple small Langerhans cell nodules in the thymus of an 11-month-old infant who had undergone thymectomy in the course of surgical correction of tetralogy of Fallot. The subtle appearance of some of these nodules and aggregates was unlike the pattern of massive thymic infiltration seen in 7 previously reported cases of thymic Langerhans cell histiocytosis in childhood. This difference led us to consider whether these aggregates might be common in the thymus but overlooked in thymus glands removed incidental to cardiac surgery. We examined histologic material from 22 sequential thymectomy specimens removed during cardiac surgery from infants and children. None (0/22) had cohesive aggregates of Langerhans cells, as were seen in the index patient's thymus. Our patient's small thymic nodules resemble more closely a pattern found in adult patients with myasthenia gravis who have been incidentally shown to have Langerhans cell histiocytosis.