PURPOSE: The purpose of the study was to evaluate the accuracy of two different fluid intake measurement methods (fluid only vs. all dietary intake) in measuring fluid balance compared to body weight change among patients with cancers. METHODS: A total of 60 cancer patients in an urban cancer center in South Korea participated in the study. Adult patients who were over 18 years old; having 24-hr I&O order; and taking either normal regular diet or soft blend diet were included. Demographic information and disease related information were also gathered. The data were analyzed using SPSS 18.0 program. RESULTS: Measuring 'fluid only' for oral intake was a more accurate measure than measuring 'all dietary intake' (p=.026 vs. p=.094). Both methods had positive correlations with the amount of weight change (r=.329, p=.010; r=.303, p=.019). Measuring body weight was a more accurate and efficient way of evaluating the fluid balance than 24 hr cumulative I&O. CONCLUSION: Developing clinical manual for selecting proper patients who needs fluid balance monitoring is imperative. Administering weight check and/or 24 hr cumulative I&O should be considered thoroughly based on solid nursing evidence in future.