Multicellular tumor spheroids are widely used as in vitro models for testing of anticancer drugs. The advantage of this approach is that it can predict the outcome of a drug treatment on human cancer cells in their natural three-dimensional environment without putting actual patients at risk. Several methods were utilized in the past to grow submillimeter-size tumor spheroids. However, these small models are not very useful for preclinical studies of tumor ablation where the goal is the complete destruction of tumors that can reach several centimeters in diameter in the human body. Here, we propose a PDMS well method for large tumor spheroid culture. Our experiments with HepG2 hepatic cancer cells show that three-dimensional aggregates of tumor cells with a volume as large as 44 mm(3) can be grown in cylindrical PDMS wells after the initial culture of tumor cells by the hanging drop method. This is a 350 times more than the maximum volume of tumor spheroids formed inside hanging drops (0.125 mm(3)). (c) 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1265-1269, 2013