The inhibitory effects of synthetic butylidenephthalide (BP) derivatives on 10% fetal bovine serum-stimulated proliferation were assayed by measuring the proliferative cell number at an interval of 12 h in primary cultures of mouse aorta smooth muscle cells (SMC). Their potencies for the anti-proliferation effect were in the order BP-42 (4,5-dihydroxy BP) > BP-92 (4,5-dihydroxy butylphthalide) > BP-97 (6,7-dihydroxy-3-(3-bromo-1-octenyl)-phthalide) > BP-82 (6,7-dihydroxy BP) > BP-86 (5,6-dihydroxy BP) > BP-87 (4,5,6-trihydroxy BP) > BP-85 (4,7-dihydroxy BP) > BP-84 (5,7-dihydroxy BP) > BP-4C3 (4-methoxy propylphthalide) > BP-7 (4-hydroxy BP) > BP-40 (4,5-dimethoxy butylphthalide) > BP-5C3 (4-hydroxy propylphthalide). We divided these anti-proliferative effects into anti-competence and anti-progression effects by using a convenient assay. BP-42 had the greatest potency in used phthalides for competence inhibition of the SMC proliferation. BP-92 had small potency for competence inhibition. BP-97 had greater potency for competence inhibition than BP-82. These results demonstrated that the anti-proliferative effect of BP-42 was greatest in used phthalides in primary cultures of vascular SMC. The 4,5-dihydroxy group and 3-butylidene or 3-(3-bromo-1-octenyl) group in these synthetic BP derivatives contributed to the anti-competence effect on SMC. BP-42 may become a prototype of an anti-atherosclerotic drug.