OBJECTIVE: To assess body hydration and the distribution of the body water compartments in defined populations of patients with inflammatory bowel disease (IBD) compared with those of matched healthy controls. SUBJECTS: Fifty-two patients with IBD at time of diagnosis (20 patients with Crohn's disease (CD-new) and 32 patients with ulcerative colitis (UC-new)), 40 patients with long-standing CD (CD-long) and 2 matched healthy control groups (n = 52 and n = 40) were recruited for the study. METHODS: Total body water (TBW) and extracellular water (ECW) were measured by deuterium oxide and bromide dilution, respectively. Intracellular water (ICW) was calculated as TBW-ECW. In addition, hydration of fat-free mass (FFM) and the ECW:ICW ratio were calculated. FFM, body fat (BF) and % body fat (%BF) were assessed by dual energy X-ray absorptiometry. RESULTS: In female IBD patients, the ECW:ICW ratio was significantly (P < 0.05) higher than in controls (CD-new: 0.89+/-0.11 vs 0.79+/-0.08, P < 0.01; UC-new: 0.85+/-0.15 vs 0.77+/-0.10, P < 0.05; CD-long: 0.86+/-0.14 vs 0.80+/-0.10, P < 0.05). In these female patients, the ICW:FFM ratio was significantly (P < 0.05) lower than in controls. Fluid shifts were especially pronounced in female patients with recently diagnosed CD. In male patients with recently diagnosed UC and in those with long-standing CD, body weight, body mass index, BF and %BF were significantly (P < 0.05) lower than in controls. No differences in body hydration or body water distribution were observed between male patients and controls. CONCLUSIONS: An altered body water distribution and body hydration was observed in female IBD patients, especially in female patients with recently diagnosed CD.