An intracavernous injection with papaverine or prostaglandin E1 (PGE1) combined with duplex ultrasonography is an objective technique to conduct a penile blood flow study (PBFS). Three hundred patients with impotence underwent papaverine (9-60 mg) induced PBFS at the University of California School of Medicine, San Francisco (UCSF), U.S.A. and another 80 patients with impotence received PGE1-induced (20 micrograms) PBFS at Veterans General Hospital-Taichung (VGH-TC), R.O.C. Preliminary evaluation of age, duration of disease and pre-injection diameter of cavernous arteries disclosed no significant difference in these 2 series. It was interesting to find that almost all parameters of vascular response to pavaverine vs PGE1 differed significantly, such as onset of response (6.5 +/- 6.5 vs 11.7 +/- 6.4 min, slower in the PGE1 group, p less than 0.001); post-injection diameter of cavernous arteries (right: 0.73 +/- 0.20 vs 0.79 +/- 0.18 mm, p = 0.03; left: 0.74 +/- 0.20 vs 0.82 +/- 0.21 mm, p = 0.005); diametral increment of cavernous arteries (right: 0.23 +/- 0.17 vs 0.33 +/- 0.17 mm, p less than 0.001; left: 0.24 +/- 0.17 vs 0.36 +/- 0.19 mm, p less than 0.001) and peak velocity (right: 27.5 +/- 16.1 vs 42.0 +/- 20.1 cm/sec, p less than 0.001; left: 28.9 +/- 15.9 vs 39.7 +/- 17.9 cm/sec, p less than 0.001). The side effects, primarily injection pain (23.8%, 19/80) in the PGE1 group and dizziness (3.0%, 9/300) in the papaverine group, were minor in these 2 series. Prolonged erection was not encountered in either series; however, immediate treatment was performed if a papaverine-induced erection lasted over 60 minutes.(ABSTRACT TRUNCATED AT 250 WORDS)