Radiotherapy (RT) is an important treatment modality in head and neck cancers. Loss of weight during RT due to various factors is a matter of concern. This study was conducted to see the pattern of weight loss and the causative factors involved. One hundred forty patients with head and neck cancer treated with radical RT, concurrent chemoradiation, or postoperative RT were retrospectively studied. A dose of 70 Gy was given in the radical and in the chemoradiation schedule. In postoperative RT, a dose of 60-64 Gy was delivered. During the weekly review of the patients, serial recording of their weight was done along with measurement of other parameters. Analysis was done to see which factors were causative in patients having a weight loss of >5 kg and of >10%. Three variables were found to be significant for the >5-kg weight loss. These were a low initial Karnofsky performance score (KPS; P < 0.001), use of chemoradiation (P < 0.001), and a total dose of >60 Gy (P = 0.04). For the >10% weight loss, the significant factors were low initial KPS (P < 0.001) and use of chemoradiation (P < 0.001). Therefore, it is important to take care of the nutrition of those patients who have a low KPS, are on chemoradiation, or will be delivered a dose of >60 Gy. The role of prophylactic Ryle's tube placement or agents such as megestrol acetate in such patients should be further investigated.