In this study we observed the inhibitory effect of Chinese herbal medicine Ganoderma lucidum (GL) on platelet aggregation in 15 healthy volunteers and 33 patients with atherosclerotic diseases. The results showed that the first and the second phase of aggregation of platelets of the healthy volunteers were obviously inhibited (P less than 0.01) when watery soluble extract of GL of different concentrations was added to the platelets in vitro, i. e., the reaction speed of platelet aggregation was slowed down. The inhibitory effect was related to dosage. Platelet aggregation induced by ADP in final concentration of 2 mumol/L and 3 mumol/L was obviously inhibited, after the patients had taken GL 1 g 3 times a day for 2 weeks, the maximum platelet aggregation inhibition rates were then 31.49% (P less than 0.01) and 17.7% (P less than 0.01) respectively. Length and weights (wet and dry) of the extracorporeal thrombi were reduced from 30.05 +/- 4.38 mm, 103.9 +/- 9.33 mg and 44.89 +/- 4.79 mg to 20.4 +/- 2.33 mm (P less than 0.05), 85.27 +/- 8.77 mg (P less than 0.01) and 35.1 +/- 4.5 mg (P less than 0.01) respectively after oral administration of GL. The results of our experiments suggested that the Chinese herbal medicine GL may be an effective inhibitory agent of platelet aggregation. However, its mechanism and active principles remain to be further investigated.