We measured paclitaxel (PTX) concentrations in blood and ascites in a patient with advanced gastric cancer treated with weekly paclitaxel (80 mg/m2). After 180 minutes from infusion of PTX, the concentration of PTX in ascites was 13 ng/ml, which was more than the reported cytotoxic dose of PTX (8.5 ng/ml). The concentration of PTX in ascites was retained, and rose rather slowly during a 24-hour period. The patient died soon after the measurement, meaning this measurement was taken at a time when the antitumor effect was ineffective and under poor general conditions. The result suggested, however, that the desired concentration of PTX in ascites can be obtained with a smaller dose of PTX in a regimen of weekly PTX.