For the purpose of alleviating pulmonary hypertension and maintaining cardiac output, which tended to be decreased to acute low-flow oxygen therapy in chronic lung diseases, we evaluated combined low-flow oxygen therapy and oral administration of OP-1206, a prostaglandin E1 derivative, in 7 patients with obstructive lung disease and 3 with restrictive one. Low flow oxygen inhalation (1 l/min. 30 minutes) increased PaO2 from 67.2 +/- 12.8 mmHg to 91.0 +/- 18.2 mmHg (p less than 0.001), PVO2 from 36.4 +/- 3.1 mmHg to 41.7 +/- 7.0 mmHg (p less than 0.01) and decreased cardiac index from 3.38 +/- 0.39 l/min/m2 to 3.07 +/- 0.32 l/min/m2 (p less than 0.01), RVSWI from 18.7 +/- 2.6 g.m/m2 to 6.1 +/- 2.0 g.m/m2 (p less than 0.01), transpulmonary driving pressure from 13.0 +/- 5.4 mmHg to 9.8 +/- 5.5 mmHg (p less than 0.01) and mean PA from 21.6 +/- 7.7 mmHg to 18.3 +/- 6.9 mmHg (p less than 0.05) with a trend of increase in COD and no change at oxygen delivery. OP-1206, on the contrary, decreased mPA from 21.6 +/- 7.7 to 18.2 +/- 5.6 mmHg (p less than 0.05), RVSWI from 8.7 +/- 2.6 g.m/m2 to 6.9 +/- 2.3 g.m/m2 (p less than 0.01) with trend of decrease in transpulmonary driving pressure, TPVR and PAR with no substantial changes in gasometric parameters. The decrease rates of PA by low-flow oxygen therapy and OP-1206 administration correlated significantly.(ABSTRACT TRUNCATED AT 250 WORDS)