Premature labor occurs in about 10% of all pregnancies and is the most frequent complication of the last half of pregnancy. At present, the etiology remains undetermined in about half of these premature labors. Treatment in these cases must therefore be symptomatic. One hundred and fifty-six patients in premature labor were treated by the use of a uterine relaxant, isoxsuprine. In 39 (25%) of the cases the course was unaffected, but in the remainder there was significant arrest of labor. In 33 (21%) labor was arrested for more than 7 days, but a premature birth followed. In 66 (42%) labor was arrested long enough for the birth of a mature infant weighing more than 2,500 gm.