In a randomized, double-blind, placebo-controlled study in patients with peripheral arterial occlusive disease stage IIb of Fontaine's classification, the efficacy of naftidrofuryl, a vasoactive substance, was investigated. Forty patients--31 men and 9 women with an average age of 62.98 +/- 10.65 years--were admitted to the study. All had a history of claudication for at least six months. The duration of the trial was eighteen weeks. After a washout period of two weeks the patients received either two times 400 mg naftidrofuryl or two identical placebo tablets daily for eight weeks. Subsequently a crossover from verum to placebo and vice versa was carried out and the alternative medication was administered for another eight weeks. Clinical results were evaluated by measuring painfree and maximal walking distance by treadmill test at a speed of 3.2 km/h and an inclination of 12 degrees. Systolic ankle pressure and brachial pressure were measured by Doppler ultrasound, and ankle/arm pressure ratio was calculated. Treadmill test, Doppler ultrasound examination, and laboratory data analysis were performed at the beginning of every study period, every four weeks during and at the end of the study. Results. The patients in both groups showed an initial homogeneity of age, risk factors, concomitant diseases, walking distance, and Doppler indices. After eight weeks naftidrofuryl resulted in a statistically significant increase in painfree (p less than 0.02) and maximal walking distance (p less than 0.05). The placebo-treated group showed only a slight, statistically nonsignificant increase in both painfree and maximal walking distance.(ABSTRACT TRUNCATED AT 250 WORDS)