ATP binding cassette A1 (ABCA1) is involved in the lipid metabolism of macrophages and has been suggested to play an important role in the development of foam cells and in the pathogenesis of atherosclerosis. We have investigated the effects of all-trans retinoic acid (atRA) on the mRNA and protein levels of ABCA1 in THP-1 cells. Our results show that both mRNA and protein levels of ABCA1 were significantly increased upon treatment with atRA. Since ABCA1 is highly regulated by liver X receptor (LXR) we also analysed the mRNA and protein expressions of LXR-alpha and LXR-beta in the THP-1 cells after treatment with atRA. Also the levels of LXR-alpha were increased by atRA. In conclusion, our results show that LXR-alpha and ABCA1 are simultaneously induced by atRA. The results also imply that the induction of ABCA1 by atRA may in part depend on the induction of LXR.