A series of 20 patients with granular lymphocyte proliferative disorders (GLPD) is reported. The criterion of inclusion was presence of persistent (> or = 6 months) granular lymphocytosis in the absence of any causative illness. Diagnoses made upon analytical control in half the patients of splenomegaly (25%) and hepatomegaly (25%) were infrequent. Clinical course was nonprogressive in 17/20 patients, but two developed high-grade NHL several years later and one showed progressive disease. Actuarial probability of survival at 5 years was 85%. Granular lymphocyte morphology was relatively homogeneous, and peripheral blood counts were preserved in the most patients. Bone marrow lymphocytic infiltration was low, as assessed by bone marrow aspiration and/or biopsy. Eosinophilia was an outstanding feature in eight cases. Ultrastructurally, all cases showed parallel tubular arrays; cytoplasmic granules and numerous short microvilli were noticed. The lymphoid phenotype was heterogeneous, the most common being CD2+CD3+CD4-CD8+, but six patients (30%) were CD4+ with variable expression of natural killer-associated antigens. Chromosomal analysis was abnormal in 4/10 patients [trisomy 19, t(5;6); inv(14) and inv(10)]. The study of beta-chain of the T-cell receptor revealed clonal rearrangements in 14 (78%), restricted to CD3+ patients (92%). In vitro culture of myeloid precursors showed decreased CFU-GM in 5/6 patients. Virological studies for HTLV-I and II were negative. In conclusion, the presence of a clonal proliferation was not correlated with the clinical course or an associated disease.