TS-1 is an oral anti-tumor drug, which contains 5-chloro-2, 4-dihydroxypyridine (CDHP), a compound mainly excreted in urine. Since the CDHP concentration is increased among patients with impaired renal function, the frequency of side effects of TS-1 increases in such patients. Therefore, we constructed a computer-aided system that enables prompt monitoring of creatinine clearance (Ccr) calculated from the serum creatinine levels of patients prescribed TS-1 at the time pharmacists prepare the medicine. With this system, we found two cases who were prescribed TS-1, despite their decreased Ccr. One was a patient whose estimated Ccr was less than 30 mL/min/m2. With such renal malfunction, pharmacokinetics of the drug was considerably changed compared with normal control, and the dosage should be reduced. The other case presented with severe jaundice and had only a mild decrease of renal function (Ccr: 50 mL/min/m2). So we measured the concentration of uracil in the urine and performed a drug lymphocyte stimulation test for further investigation of concomitant affecting factors. Our system is useful because it can show pharmacists both the dosage TS-1 patients take and their renal function at a glance in real time. This system can be adapted for every medicine which might accumulate in patients with renal dysfunction.