Recent studies showed that statins reduce ATP-binding cassette transporter A1 expression and ATP-binding cassette transporter A1–mediated cholesterol efflux in macrophages, whereas the effect of statins on ATP-binding cassette transporter G1 (ABCG1), another important lipid transporter, is still unclear. Here, we investigated the expression and functionality of ABCG1 on statins in THP-1–derived macrophages and human peripheral blood mononuclear cells. Simvastatin and atorvastatin were used in this study. Treatment with statins significantly decreased ABCG1-mediated cholesterol efflux in human macrophages (from 33.8% ± 2.8% of control to 22.9% ± 1.7% of 10 µM simvastatin or to 23.3% ± 3.3% of 10 µM atorvastatin; P < 0.01, n = 4), whereas the protein expression of ABCG1 remained unaltered on statins. Further analysis revealed that 2 major ABCG1 isoforms responded to statins differently. The expression of ABCG1-S, which exhibited higher activity in cholesterol efflux than ABCG1-L, was significantly decreased on statins compared with increased expression of ABCG1-L. The results suggested that the proportion change of ABCG1 isoform expressions could contribute to reduced ABCG1 functionality under treatment with statins. The effects of statins on ABCG1 isoform expression and functionality were reversed by low-dose liver X receptor agonist, TO-901317, indicating that statins' downregulation of ABCG1 functionality was likely through liver X receptor–dependent pathway. In conclusion, simvastatin and atorvastatin decreased ABCG1-mediated cholesterol efflux in human macrophages without alteration of total ABCG1 protein level. The proportion change of ABCG1 isoforms expressions may be involved in the down-regulation of ABCG1 functionality by statins, which provided a novel mechanism for the regulation of ABCG1 activity.