Ten patients with the Pickwickian syndrome, characterized by obesity, hypoxemia, hypercapnia, polycythemia, and cor pulmonale, underwent long-term treatment as outpatients with medroxyprogesterone acetate. Although there was no significant weight change in the group, PaO2 rose 12.6 +/- 2.7 mm Hg (SEM) from 49 +/- 2.6 mm Hg to 62 +/- 2.3 mm Hg (P less than 0.001), while PaCO2 fell 13 +/- 2.6 mm Hg from 51 +/- 1.9 mm Hg to 38 +/- 1.2 mm Hg (P less than 0.001). Hematocrit fell from 56 +/- 2.5% to 50 +/- 1.2%, a mean fall of 6% (P less than 0.01), during medroxyprogesterone acetate therapy. In the 2 patients who had cardiac catheterization before and during medroxyprogesterone acetate therapy, mean pulmonary arterial pressure fell 13 and 19 mm Hg. There were no recurrences of cor pulmonale during treatment. These effects on arterial blood gas values and clinical state were sustained during therapy. On withdrawal of medroxyprogesterone acetate during 1-month period, arterial oxygen and carbon dioxide tensions deteriorated to their previous pretreatment values. Reinstitution of medroxyprogesterone acetate caused improvement in both the oxygen and carbon dioxide tensions. We conclude that sublingual medroxyprogesterone acetate therapy is useful in the management of the Pickwickian syndrome.