In Northern Ireland, with a population of 1.5 million, thirteen cases of Letterer-Siwe disease have been diagnosed over the past 21 years. The average age of onset was 10.5 months. There was a 69% mortality with an average survival time from diagnosis to death of 3.3 months. Four children survived with no morbidity. All deaths were from pulmonary complications, but two cases with pulmonary infiltration responded to treatment with quadruple chemotherapy. There were no familial cases in this study.