Purpose: To identify the risk factors for moderate to severe subcutaneous fibrosis after whole neck irradiation. Methods and Materials: We analyzed 233 cases of patients who had undergone whole neck irradiation with 4-MV X-ray or 8–10-MeV electrons, or both, and had been followed with regard to their skin condition for at least 1 year. The prescribed dose to the whole neck ranged from 19.2 to 72.4 Gy (median 50). The skin-absorbed dose was specified as that at a depth of 4.1 mm (d4.1-mmdepth), and a biologically equivalent dose (BED) of d4.1-mmdepth was also estimated (BED1.8 4.1-mmdepth). Results: Univariate analysis revealed that previous neck dissection, concurrent chemotherapy, corticosteroid administration as a part of chemotherapy, fractionation, and BED1.8 4.1-mmdepth were significant prognostic variables. Multivariate analysis showed that BED1.8 4.1-mmdepth and previous neck dissection were the only prognostic variables for moderate to severe subcutaneous fibrosis. Conclusion: A high dose to a 4.1-mm depth of the skin and a history of neck dissection were identified as the predominant risk factors for moderate to severe subcutaneous fibrosis after whole neck irradiation. A subcutaneous dose should be considered in radiotherapy treatment planning involving the whole neck, especially in cases in which patients have undergone previous neck dissection.