Weight loss during cancer chemotherapy affects the continuation of treatment; therefore, it is important to maintain and improve nutritional status. Additionally, appropriate fluid and electrolyte replacement is essential for maintaining life. This study included 100 patients who underwent outpatient chemotherapy in April 2021. The degree of dehydration was assessed based on serum osmolality, and the possibility of screening was examined by a hidden dehydration check sheet. Hidden dehydration was noted in 38 patients and dehydration in 6 patients. The incidence of pancreatic cancer was significantly lower than that of lung cancer. In the hidden dehydration check sheet, 51 patients were found to present with high possibility of hidden dehydration and required consultation to a medical professional. The serum osmolality of the results was not significantly different. During outpatient cancer chemotherapy, a certain percentage of patients present with hidden dehydration. To detect dehydration at an early stage, serum osmolality should be actively measured and continuous diet counseling, including confirmation of food and fluid intake, is required.