Melanomas can be difficult to diagnose histologically if they deviate in their growth pattern or cytology only minimally from a nevus. On occasion, even experts on melanocytic lesions may not reach a consensus on whether a lesion is a benign but unusual nevus or a malignant melanoma mimicking a nevus. This diagnostic dilemma is particularly well known for the distinction of Spitz nevus from melanoma. Diagnostic uncertainty and disagreement among consultant pathologists lead to confusion about the prognosis and clinical management of patients. In this study we present the clinical and pathologic findings of 10 patients with diagnostically controversial melanocytic tumors, who underwent sentinel lymph node biopsy. In all of these cases, the diagnostic controversy among experts was between Spitz nevus and melanoma. Seven patients were female, and three were male, ranging in age from 7 to 46 years (mean 21 years). Histologic examination of the sentinel lymph nodes revealed tumor deposits in the lymph node parenchyma in 5 of 10 patients. Among patients with positive sentinel lymph nodes, two had satellite nodules and one showed additional tumor deposits in three nonsentinel regional lymph nodes. All patients are alive and free of disease with a follow-up of 10-54 months (mean 34 months). Our study illustrates the role of a sentinel lymph node biopsy in the evaluation of patients with diagnostically controversial melanocytic tumors. Although the presence of metastatic tumor deposits in the sentinel lymph node supports the diagnosis of malignant melanoma, further studies are needed to determine the prognostic significance of the sentinel lymph node findings in such patients.