Structure-activity relationships for a series of 3-phenoxy-1,4-diarylazetidin-2-ones were investigated, leading to the discovery of a number of potent antiproliferative compounds, including trans-4-(3-hydroxy-4-methoxyphenyl)-3-phenoxy-1-(3,4,5-trimethoxyphenyl)azetidin-2-one (78b) and trans-4-(3-amino-4-methoxyphenyl)-3-phenoxy-1-(3,4,5-trimethoxyphenyl)azetidin-2-one (90b). X-ray crystallography studies indicate the potential importance of the torsional angle between the 1-phenyl "A" ring and 4-phenyl "B" ring for potent antiproliferative activity and that a trans configuration between the 3-phenoxy and 4-phenyl rings is generally optimal. These compounds displayed IC50 values of 38 and 19 nM, respectively, in MCF-7 breast cancer cells, inhibited the polymerization of isolated tubulin in vitro, disrupted the microtubular structure in MCF-7 cells as visualized by confocal microscopy, and caused G2/M arrest and apoptosis. Compound 90b possessed a mean GI50 value of 22 nM in the NCI60 cell line screen, displayed minimal cytotoxicity, and was shown to interact at the colchicine-binding site on β-tubulin. Phosphate and amino acid prodrugs of both 78b and 90b were synthesized, of which the alanine amide 102b retained potency and is a promising candidate for further clinical development.