Experimental and human studies have provided evidence that a high intake of n-6 polyunsaturated fatty acids stimulates mammary carcinogenesis. Arachidonic acid, an n-6fatty acid consumed in the diet or derived from dietary linoleic acid, is thought to play a key role in enhancement of mammary tumor development. In this study, we investigated the effects of arachidonic acid on T-47D breast cancer cell growth, cell cycle progression, and the expression of cyclin D1 mRNA. Our data show that arachidonic acid stimulated the growth of T-47D cells with a twofold stimulation at 5 microg/ml. This effect was associated with an increase in the proportion of cells in the S phase of the cell cycle and preceded by stimulation of the expression of cyclin D1 mRNA, with maximal induction at 5 microg/ml. Cyclin D1 mRNA levels were increased within two hours of treatment and were maximal at five hours. These results suggest that arachidonic acid may exert a stimulatory effect on breast cancer cell growth and that this effect possibly involves the induction of cyclin D1 gene expression leading to cell cycle progression.