Background and Objective: Estrogen modulates inflammatory responses, but the mechanisms involved have not yet been identified. Periodontal ligament (PDL) cells produce chemokines (a group of chemoattractant molecules that recruit leukocytes) and it has been suggested that estrogen modulates periodontal inflammation by regulating the expression of chemokines by PDL cells. Therefore, the objectives of this study were to investigate the regulation of chemokine ligand 2 [CCL2/monocyte chemoattractant protein 1 (MCP-1)], chemokine ligand 3 [CCL3/macrophage inflammatory protein-1alpha (MIP-1alpha)] and chemokine ligand 5 (CCL5/RANTES) by estrogen in human PDL cells. Material and Methods: PDL cells were obtained from the PDL of premolars, extracted for orthodontic reasons, from two boys and two girls (16 and 17 years of age). PDL cell CCL2, CCL3 and CCL5 mRNA transcripts were determined by quantitative real-time PCR. The concentrations of CCL2, CCL3 and CCL5 proteins were determined by ELISAs. Results: Treatment with 0.5 mug/mL of lipopolysaccharide (LPS, from Escherichia coli) + 100 nm 17beta-estradiol (E(2)) for 24 h reduced the expression of CCL3 mRNA by about 40% compared to PDL cells treated with LPS alone. Attenuation of CCL3 mRNA was not associated with a decrease in CCL3 protein within 48 h, suggesting a slow turnover of the CCL3 protein. Interindividual differences in the effects of E(2) on CCL5 mRNA expression were observed. E(2) (100 nm) increased the expression of CCL5 by 40-60% in PDL cells derived from two subjects but reduced the expression of CCL5 by about 30% in cells from another subject. CCL2 mRNA and CCL2 protein were highly expressed, but not regulated by E(2). Similar data were observed in cells obtained from both boys and girls. Conclusion: Regulation, by estrogen, of chemokine expression in PDL cells shows a complex pattern involving the down-regulation as well as the up-regulation of chemokines, suggesting that estrogen exerts both anti-inflammatory and proinflammatory effects through these mechanisms.