A complete three-dimensional X-ray crystal structure analysis of 1,1-bis(4-hydroxyphenyl)-2-phenylethene (BHPE) has been carried out. Reflexes were collected at room temperature. After isotropic refinement ofF-values by least-squares,Ris 0.163.BHPEcrystallizes with 8 molecules in a unit cell of monoclinic symmetry, space group C2/c and cell dimensionsa=20.851,b=15.505,c=10.536 Å, β=107.54°. The molecule ofBHPEis not flat, the aromatic rings are twisted out of the ethene plane with angles of −30.16° (ring B), −51.45° (ring C) and −33.49° (ring A). The bond angle between the 1,1-standing, 4-hydroxy-substituted phenyl rings amounts to 115.3° resulting in a distance between the hydroxy groups of 9.636 Å.BHPEproved to be a weak “impeded” estrogen with minor antiestrogenic potency, though its estrogen receptor affinity is very high (29%, estradiol 100%). A discussion of the influence of the spatial structure ofBHPEand related substances on its estrogenic/antiestrogenic and mammary tumor-inhibiting