We present the cytogenetic investigations of five histiocytic tumour lesions from children. In four cases there was a confirmed diagnosis of Langerhans cell histiocytosis (LCH) and one case of histiocytosis that did not fulfil all the criteria for true LCH. All five cases showed cytogenetic abnormalities, including the first report of an abnormal clone in LCH. The clone showed a t(7;12)(q11.2;p13) translocation and was detected in only a small percentage of cells. This case and a further three also contained non-clonal abnormalities and an increase in chromosome breakage. The fifth case, the only one in which no acquired abnormalities were seen, had a constitutional paracentric inversion of chromosome 13q.