In a prospective study, CD45 isoform expression on T lymphocytes was analysed in seven patients with haemophagocytic lymphohistiocytosis (HLH) and their family members. Six patients, their parents and seven healthy age-matched controls showed the normal pattern of three subpopulations of CD45R expression. In one patient, his mother and one of his healthy brothers only two subpopulations could be identified, which might indicate that the maturation from a CD45RA/RO double-positive to a CD45RO single-positive phenotype does not occur in these individuals. It is not clear if this finding had any impact on the development of HLH in this patient and if the pattern of CD45RA and RO expression observed in this family might represent a predisposition to HLH. In another patient a marked increase of the CD45RO single-positive T cell population was observed during a period of increased disease activity. The clinical relevance of this observation and whether CD45RO expression could serve as a marker for disease activity, or in selected individuals as a susceptibility marker for HLH, remain unknown.