Objective: Investigation of possible effects of Silybum marianum extract (SME) on adenosine deaminase (ADA) activity in cancerous and noncancerous human gastric and colon tissues to obtain information about possible mechanism of anticancer action of S. marianum. Materials and Methods: Cancerous and noncancerous human gastric and colon tissues removed from patients by surgical operations were used in the studies. The extract was prepared in distilled water. Before and after treatment with the extract, ADA activities in the samples were measured. Results: ADA activity was found to be lowered significantly in cancerous gastric tissues but not in noncancerous gastric tissues after treatment with the SME. In the colon tissues, ADA activities were however found to increase after the treatment of SME. Conclusion: Our results suggest that the aqueous extract from S. marianum inhibits ADA activity in cancerous gastric tissues significantly. It is suggested that in addition to other proposed mechanisms, accumulated adenosine due to the inhibition of ADA might also play a part in the anticancer properties of the S. marianum.