Patients have experienced severe breathlessness during second trimester abortion initiated by the intramuscular injection of 15-methyl-prostaglandin F2α (15-me-PGF2α). In four healthy women given 400 μg of 15-me-PGF2α to induce abortion, pulmonary function tests showed reductions in arterial oxygen tension, maximum expired air flow and vital capacity. Residual lung volume and the slope of phase III of the closing volume curves increased. These indications of marked peripheral bronchoconstriction persisted for more than 1 hour. In eight other women requesting abortion, initial pulmonary arterial pressures (11 ± 1 mm Hg) were less than normal (16 ± 1 mm Hg) suggesting that the lung vessels were dilated during pregnancy. The 15-me-PGF2α caused only a modest increase in pulmonary arterial and wedge pressures. Thus, bronchoconstriction rather than pulmonary circulatory changes probably caused the breathlessness observed in some patients. As an intramuscular abortifacient, 15-me-PGF2α has a widespread potential use, but it should not be given to patients with a history of asthma.