Nasopharynx cancer (NPC) occurs frequently in southern China. Radiotherapy (RT) is the standard treatment for this cancer, and weight loss is commonly observed during and for a prolonged period after RT. The determinants of this phenomenon are not well known. The nutritional status of 38 NPC patients was assessed serially before and for a 6-mo period after RT. Body weight, body composition (by dual X-ray absorptiometry), basal metabolic rate (BMR, by indirect calorimetry), and calorie intake (by 3-day dietary record) were documented at pre-RT (T0), end-RT (T1), 2 mo post-end-RT (T2), and 6 mo post-end-RT (T3). The BMI at end-RT was 21.5 +- 3.7 kg/m2 (range = 13.7-27.9 kg/m2) and was significantly lower than that at pre-RT (P < 0.001). Body weight at T1-T3 was significantly lower than that at T0 (P < 0.001). Mean percentage weight loss was 10.8% at end-RT. Fifty-five percent of patients (20 of 30) had 10% weight loss by the end of RT. BMR corrected for body weight did not change significantly among the four time points, but BMRs corrected for lean body mass at T2 and T3 were significantly lower than that at T0 (P < 0.01). Patients' calorie intake generally declined from T0 to T1 (from 1,857 P < 411 kcal to 11,68 +- 549 kcal, P < 0.001) and then increased from T1 to T3. The calorie intakes at T2 and T3 were not significantly different from that at T0 but were significantly higher than that at T1. Patients were in negative energy balance before, during, and up until 6 mo after RT. The recovery in body weight lagged behind the recovery of dietary intake. Eighty-two percent of NPC patients had significant weight loss and was in negative energy balance at the end of RT, which persisted for at least 6 mo. This finding suggests that there is room for improvement in the nutritional status of patients with NPC treated with RT.