The authors evaluated serum levels of folate, homocysteine, and vitamin B(12) in patients with head and neck squamous cell carcinoma (HNSCC) and in patients with laryngeal leukoplakia, a well known preneoplastic lesion. One hundred forty-four consecutive, untreated patients with HNSCC and 40 consecutive, untreated patients with laryngeal leukoplakia were enrolled in the Department of Otolaryngology at the authors' institution. Data from those patients were compared with data from one control group, which included 90 smokers, and from another control group, which included 120 nonsmokers. Serum levels of homocysteine, folate, and vitamin B(12) were measured by an automated immunoassay method based on fluorescence polarization immunoassay technology. Comparing groups by Student-Newman-Keuls test, serum folate levels were significantly lower in patients with HNSCC and in patients with laryngeal leukoplakia compared with serum folate levels in both the smoker control group and the nonsmoker control group. Serum homocysteine levels in patients with HNSCC were significantly higher compared with homocysteine levels both in the smoker and nonsmoker control groups and in patients with laryngeal leukoplakia. There were no statistically significant differences between groups in serum vitamin B(12) levels. A role for folate deficiency as a risk factor in head and neck carcinogenesis is plausible. A chemoprevention protocol with folate is both feasible and ethically correct and is in progress at the authors' institution. Homocysteine levels in patients with HNSCC probably are affected largely by the HNSCC phenotype. An accumulation of homocysteine may reveal a genetic defect, which, theoretically, may be a target for pharmacologic therapy, for example, with antifolic drugs. (c) 2004 American Cancer Society.