Cladosporol was isolated from the fermentation broth of Alternaria alternata var. monosporus obtained from the inner bark of the yew tree and mutated for many generations. We investigated the antitumor effects of cladosporol in vitro and in vivo. The growth-inhibitory effects of cladosporol in vitro against six human cancer cell lines were examined using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. The results showed that cladosporol selectively killed cancer cells and had a significant inhibitory effect on the human gastric carcinoma cell line MGC-803 in a concentration- and time-dependent manner. In vivo, cladosporol also showed antitumor activity in nude mice bearing MGC-803 gastric cancer xenografts. These findings suggest that cladosporol has potentially useful growth inhibitory effects on human gastric carcinoma cell lines.