INTRODUCTION: We wished to determine bioelectrical impedance (BIA) correlates before hydration or changes during hydration and determine if these changes were prognostically important. METHODS AND MATERIALS: Fifty eligible patients underwent BIA measurements 3 consecutive days. Laboratory studies (electrolytes, creatinine, and hemoglobin) on day 1; weights and vital signs were recorded. Kaplan-Meier survival estimates were made at 30 and 60 days. Hazard ratios (HRs) based on Cox proportional hazards model were calculated. RESULTS: Weight loss was associated with shorter survival. A higher phase angle (PA) on day 1 predicted longer survival. Increased PA during hydration predicted shorter survival: increased weight during hydration predicted longer survival. DISCUSSION: Higher phase angle before hydration predicts poorer survival and, paradoxically, an increase in phase angle during hydration predicted poorer survival and preexisting intracellular dehydration, cachexia, or poor membrane function. CONCLUSIONS: Phase angle and weight during hydration predict survival in cancer.