We studied a consecutive series of 78 stage I cutaneous malignant melanoma in order to identify variables which might predict development of metastases. Anatomical site, sex, tumor thickness, Clark level, microscopic ulceration, growth phase, histologic type, cell type, and DNA ploidy were investigated. Lesions with tumor thickness 1.5 mm, Clark level IV-V, microscopic ulceration and DNA aneuploidy were at high risk for the development of metastases. This study showed the prognostic importance of DNA ploidy in stage I cutaneous malignant melanoma and the strong relationship between DNA ploidy and classic prognostic factors. This variable can be used in routine diagnosis for selecting a high-risk group of patients who may benefit from a more aggressive therapeutic approach.