Haematogones have been extensively characterized in bone marrow, but not in the peripheral blood (PB). We studied 102 PB samples from adult patients with a sensitive flow cytometry method. Sixty-six of 102 samples (65%) contained detectable haematogones, ranging from 0·01% to 1·3% of white blood cells (median 0·06%, mean 0·13%). Of 66 cases with complete blood count data, 51 had absolute haematogone counts of 0·00037–0·105 × 109/l (median 0·0054 × 109/l, mean 0·012 × 109/l). PB haematogones belonged exclusively to the most mature maturational stage. These findings have implications for PB analysis of minimal residual disease in acute lymphoblastic leukaemia and follicular lymphoma.