The possible role of sphingomyelin cycle for the regulation of cell proliferation was investigated in human keratinocytes. The time-dependent breakdown of sphingomyelin was observed in the immortalized human keratinocyte cell line HaCaT as well as in primary human keratinocytes thereby providing evidence that the sphingomyelin cycle might be of importance in the epidermis. Peak levels of 20-30% sphingomyelin hydrolysis were measured 3 h after treatment of the cells with 1 alpha,25-dihydroxyvitamin D3 or with the vitamin D3 analogue, calcipotriol. The decrease of sphingomyelin upon addition of vitamin D3 or calcipotriol was accompanied by an approximately 70% increase of ceramide in the cells. The effects of vitamin D3 and calcipotriol on sphingomyelin breakdown were paralleled by their antiproliferative potency. Furthermore, the cell-permeable ceramide, N-acetylsphingosine, and natural ceramide inhibited cell proliferation of human keratinocytes. The results presented suggest that induction of the sphingomyelin cycle represents one mechanism mediating the therapeutic effect of calcipotriol in treatment of psoriasis.