Purpose: The purpose of this study was to identify differences between intake and output balance and body weight changes and to identify factors related to differences in critically ill patients. Methods: The participants for this descriptive correlational study were 65 medical surgical ICU patients. The data were collected from patient medical records. Results: Mean age of the patients was 63.80 years (${pm}15.21$). Body weight changes for 48 hours averaged 281.54g (${pm}2210.48$). I&O balance for 48 hours corrected for insensible loss averaged 398.1ml. Differences ranged from 45mL to 7,535mL. In the distribution of absolute difference between body weight change and intake and output balance, only 40% of the patients were within less than 1,000 mL. Factors relating to accurate measure of intake and output were ventilation methods, respiration patterns, and edema status. Conclusion: Although mean values of weight change and I&O balance for all patients were very close, the range of differences was very wide indicating that, for many patients, intake and output is not an appropriate indicator of body fluid balance. Therefore, because of the frequency fever and/or hyperventilation, nurses need to use caution when using intake and output balance only to estimate current body fluid status for critically ill patients.