In Southeast Asia and Taiwan, betel quid chewing is prevalent. Patients with head and neck cancer who chewed betel quid habitually seem to experience more severe chemotherapy-induced mucositis in our clinical practice. To validate this issue, patients with untreated head and neck cancer who received cisplatin (cDDP) plus a 5-fluorouracil (5-FU)-based neoadjuvant chemotherapy were included in this analysis. Information on the consumption of betel quid, tobacco, and alcohol were recorded before chemotherapy. Oral submucous fibrosis (OSF) was diagnosed clinically according to the fibrotic appearance of the mucosa and trismus. Mucositis was scored according to the World Health Organization criteria, and the mucositis score of the first course of chemotherapy was used for analysis. From December 1993 to April 1996, 120 patients were enrolled in this trial. Neither the betel quid chewing nor the cancer of the oral cavity was to be a significant factor for mucositis. However, clinically diagnosed OSF was found to display a significant correlation with more severe mucositis (p = 0.02). We concluded that in betel quid chewing-prevalent areas, OSF was a risk factor of more severe mucositis in head and neck cancer patients treated by CDDP and 5-FU-based regimens.