Body weight and fluid input/output are usually monitored for checking fluid balance in case of intravenous hyperhydration during nephrotoxic chemotherapy. The reliability of measuring fluid input/output is uncertain. Moreover, this measurement is redundant, complex, labour-intensive and represents an occupational hazard for nurses and other health-care workers handling fluids or body excreta. In a prospective cohort study, we determined the concordance between body weight and fluid intake/output. We also examined the clinical consequences with respect to the safety of selecting only body weight measurement as a parameter for fluid overload. A total of 591 combined observations of fluid balances and body weights were collected. We observed a higher increase in body weight than in fluid balance. The Pearson correlation between fluid balance and body weight was relatively low (r = 0.28). With regard to the safety of measuring body weight only, we found 4 cases (0.6%) who might not have received furosemide if the fluid input/output had not been measured, without clinical consequences, however. After standardization, body weight can safely be used as the only parameter for monitoring fluid retention in case of hyperhydration during chemotherapy.