Patients with thin primary melanomas (< or = 1 mm) generally have an excellent prognosis. However, the presence of a vertical growth phase (VGP) adversely impacts the survival rate. We report on the rate of occurrence of nodal metastasis in patients with thin primary melanomas with a VGP who are offered sentinel lymph node (SLN) biopsy. Among 235 patients with clinically localized cutaneous melanomas who underwent successful SLN biopsy, 71 had lesions 1 mm or smaller with a VGP. The SLN was localized by using blue dye and a radiotracer. If negative for tumor by using hematoxylin and eosin staining, the SLN was further examined by immunohistochemistry. The rate of occurrence of SLN metastasis was 15.2% in patients with melanomas deeper than 1 mm and 5.6% in patients with thin melanomas. Three patients with thin melanomas and a positive SLN had low-risk lesions, based on a highly accurate six-variable multivariate logistic regression model for predicting 8-year survival in stage I/II melanomas. The fourth patient had a low- to intermediate-risk lesion based on this model. At the time of the lymphadenectomy, one patient had two additional nodes with metastasis. VGP in a melanoma 1 mm or smaller seems to be a risk factor for nodal metastasis. The risk of nodal disease may not be accurately predicted by the use of a multivariate logistic regression model that incorporates thickness, mitotic rate, regression, tumor-infiltrating lymphocytes, sex, and anatomical site. Patients with thin lesions having VGP should be evaluated for SLN biopsy and trials of adjuvant therapy when stage III disease is found.