Objective To investigate the effects of different doses of intravenous domestic urokinase. Methods One thousand and twenty three cases of acute myocardial infarction within 6 hours after acute onset were divided randomly into two groups with 539 cases in the low dose group (urokinase 22 000IU/kg, total dose not lower than 1.2 million) and 484 in the high dose group (30 000IU/kg, total dose not more than 2.4 million). The patency of infarct-related coronary artery was assessed by uniform clinical criteria. Results In the low and high dose groups the patency rates were 67.3% and 67.8% respectively (P > 0.05); 4-week mortalities were 9.5% and 8.7% respectively (P > 0.05). Frequency of severe complications had no statistically significant difference between the two groups. Adverse bleeding was a little more frequent in the high than in the low dose group, and fatal cerebral hemorrhage in 2 cases occurred in the high dose group. Conclusion Low dose urokinase, 22 000 IU per kg body weight with a total dose not lower than 1.2 million IU, is not only equally effective as high dose but also safer.