Uncaria tomentosa ("Uña de gato") (Rubiaceae) is widely used in South America for treatment of gastritis, arthritis, cancer and inflammatory conditions. Recent literature reports cytostatic, antiproliferative, anti-inflammatory, mutagenic and anti-mutagenic properties of extracts of the plant. The present study investigates the possible proapoptotic mechanism via the activation of caspase3, in cytostatic effects of root bark extracts of Uncaria tomentosa on three different tumoral cell lines.