This study examined the clonality of B- and T-cells by PCR in 83 patients with Philadelphia-negative myeloproliferative disorders (Ph-MPD), to investigate its clinical and morphological correlates. Clonal lymphocytic populations were found in 23% of patients (T: n = 20, B: n = 3), with no frequency differences between ET, CIMF and PV. At the presentation, patients with clonal bands were older (58.1+/-13.8 vs 47.5+/-14.6, p = 0.0039), but did not differ in other clinical parameters. After the median follow-up of 21 months they were less likely to be asymptomatic (11.8% vs 41.1%, p = 0.029). The T-cell clonality was the strongest predictor of the symptomatic last follow-up by discriminant function analysis, surpassing the patient's age. This surprising negative prognostic impact of lymphocyte clonality in Ph-MPD may result from this phenomenon to be a better measure of the 'hematopoietic biologic age' than the metrical age itself.