PURPOSE: To evaluate the novel tumor biomarker YKL-40 in serial serum samples from patients with American Joint Committee on Cancer (AJCC) stage I and II melanoma from the time of diagnosis and during routine follow-up. Macrophages, neutrophils, and cancer cells secrete YKL-40, and a high serum level has been associated with poor prognosis in patients with several cancer types. PATIENTS AND METHODS: Serum samples from 234 patients with stage I (n = 162) and II (n = 72) melanoma were analyzed for YKL-40 by enzyme-linked immunosorbent assay. Serial samples were obtained before definitive primary surgery and during follow-up. RESULTS: After a median follow-up period of 66 months (range, 1 to 97 months), 41 relapses (18%) and 39 deaths (17%) were observed. Serum YKL-40 treated as an updated continuous covariate were analyzed together with the covariates sex, age, primary tumor site, ulceration, thickness, Clark level and histologic subtype in a Cox proportional hazard model. Serum YKL-40 was an independent prognostic factor of relapse-free survival (hazard ratio [HR], 1.6; 95% CI, 1.1 to 2.5; P = .03) and overall survival (HR, 1.8; 95% CI, 1.2 to 2.6; P = .002) together with thickness and ulceration. The serum level of YKL-40 (dichotomized as normal or elevated) at the time of diagnosis was also an independent prognostic factor for overall survival (HR, 3.6, 95% CI, 1.7 to 7.7; P = .001). CONCLUSION: Serum YKL-40 may be an early biomarker of relapse and survival in patients with AJCC stage I and II melanoma. Serum YKL-40 may also be useful for patient stratification and follow-up in clinical trials. Our results need confirmation in an independent study.