BACKGROUND: Irinotecan (CPT-11) is the key drug used in chemotherapy for many malignant tumors. CPT-11 has cholinergic activity and induces perspiration during intravenous administration. In this study, concentrations of CPT-11 and its active metabolite, SN-38, released during perspiration were measured and risk of exposure of these drugs was assessed. METHOD: Beads of sweat were collected using a dropper from four patients undergoing a chemotherapy regimen involving intravenous administration of CPT-11. The concentrations of CPT-11 and SN-38 in sweat were measured using liquid chromatography tandem mass spectrometry. RESULT: Chemotherapy regimens were capecitabine and irinotecan plus bevacizumab (n = 1), CPT-11 monotherapy (n = 1), and oxaliplatin-irinotecan-leucovorin-5-fluorouracil (n = 2). Uridine diphosphate-glucuronosyltransferase 1A1 phenotypes were *6 homo-type (n = 1), *6 hetero-type (n = 1), and wild type (n = 2). CPT-11 dose was 292.3 ± 75.5 mg/body weight (mean ± standard deviation). CPT-11 was detected in sweat secreted by all the four patients, and its mean (±standard deviation) concentration was 252.6 (±111.9) ng/ml. SN-38 was detected in only one of the patients who received oxaliplatin-irinotecan-leucovorin-5-fluorouracil treatment and who had the wild-type uridine diphosphate-glucuronosyltransferase 1A1 phenotype at a concentration of 74.37 ng/ml. CONCLUSION: CPT-11 and SN-38 are detected in sweat released during intravenous CPT-11 administration. Beads of sweat or linen clothes that absorb the sweat might be the source of CPT-11 and SN-38 exposure.