Lymphoid aggregates (LA) are a common finding in bone marrow biopsies but little is known about their clinical implications and biological significance. We found LA in 51/206 patients with myelodysplastic syndromes (MDS). There was no correlation with age, disease progression or overall survival. The group with LA had lower hemoglobin values (P=0.03), and was associated with an increase in reticulin fibres (P=0.01). Although they were more frequent in RAEB, this did not reach statistical significance. Most LA had a benign morphology and showed CD20 expression in three distinct patterns: central, perinodular or diffuse. No evidence of an association with lymphoproliferative disease was observed. LA probably represent an ongoing immune stimulation and are probably related to an altered bone marrow microenvironment, with no impact on prognosis.