Two series of triazaspiroalkanedienes, bearing a substituted phenoxy propyloxy side chain, were identified as potent mammalian DHFR inhibitors. One series has a 6,5-spiro bicyclic ring system and the other series has a 6,6-spiro bicyclic system. Both series were synthesized and tested for in vitro mammalian DHFR inhibitory activity and antiproliferative activity against A549 human lung-cancer cells. Compound 3c showed the highest antiproliferative activity against A549 cells with an IC(50) value of 27.1nM. Rescue experiment confirmed its antifolate antiproliferative mechanism. The excellent antifolate and antiproliferative activity of selected analogues presented in this study warrants further investigation as potential leads in the anticancer drug discovery.