A recent analysis of literature reports of familial clusters of chronic lymphocytic leukaemia (CLL) suggested that affected offspring are diagnosed at an age 21 years less than CLL parents. Such an analysis risks sampling bias. We avoided these potential sources of bias by systematic ascertainment of CLL families. Statistical analysis of 10 such families showed a significant decline of 22 years between the mean ages at diagnosis of disease in parents and offspring. This confirms the analysis of literature reports and provides the first systematic investigation of a phenomenon which, if familial clustering of CLL cases is considered due to genetic effects, points to familial CLL manifesting anticipation.