This invention discloses 17,20(Z)-dehydro vitamin D analogs, and specifically 17(Z)-1.alpha.,25-dihydroxy-17(20)-dehydro-2-methylene-19-nor-vitamin D.sub.3 and pharmaceutical uses therefor. This compound exhibits pronounced activity in arresting the proliferation of undifferentiated cells and inducing their differentiation to the monocyte thus evidencing use as an anti-cancer agent and for the treatment of skin diseases such as psoriasis as well as skin conditions such as wrinkles, slack skin, dry skin and insufficient sebum secretion. This compound also may be used to treat autoimmune disorders and inflammatory diseases in humans as well as renal osteodystrophy and obesity. This compound also has significant calcemic activity making it a therapeutic agent for the treatment or prophylaxis of osteoporosis, osteomalacia, renal osteodystrophy and hypoparathyroidism.