Summary. Anticipation, a phenomenon in which an inherited disease is diagnosed at an earlier age in each successive generation of a family, has been demonstrated in certain heritable neurological disorders and in multiple myeloma, non-Hodgkin's lymphoma and other haematological neoplasms. The present study was conducted to determine whether anticipation occurs in familial chronic lymphocytic leukaemia (CLL). Fourteen published reports of multigenerational familial CLL were analysed for anticipation, together with 10 previously unreported families with familial CLL, and the difference in disease-free survival between generations was determined. The difference between age at onset for each affected parent±child pair was tested against the null hypothesis that there was no difference in age at onset. The age at onset of the studied cases was also compared with that of the Surveillance Epidemiology and End Results (SEER) Program of the U.S. National Cancer Institute. The median ages at onset in the child and parent generations of all families (51´0 and 72´0 years respectively) were significantly different (P , 0´000001), and the null hypothesis was rejected (P , 0´000001). A significant difference was observed between the ages of onset of the child generation and the SEER population (P , 0´00001), but not between the parent generation and the SEER population. Anticipation characterizes familial CLL.