Thin (<or=0.76 mm) level II cutaneous superficial spreading melanomas (SSMs) are known to be of excellent prognosis and very few recur, metastasize, or are lethal. Although many prognostic features at this stage have been studied, none appears to be statistically significant. The concept of tumor growth phase is correlated with Clark's level except for level II. SSM level II show either an invasive vertical growth phase or an invasive radial growth phase. The aim of our study (retrospective, multicenter, and case-control type) was to investigate the prognostic impact of vertical growth phase in thin level II cutaneous SSM. We identified 12 patients of poor outcome with complete initial excision. Each case was matched with three controls for gender, age, location, tumor thickness, and follow-up period since diagnosis. Independent pathologists studied all cases and controls. Univariate analyses were performed with a conditional logistic regression method. A kappa test was used to assess reproducibility between pathologists. Our study is the first and largest that shows that vertical growth phase is the only statistically significant prognostic factor for thin level II cutaneous SSM. We propose that growth phase evaluation (a minimum of eight serial sections being mandatory not to underdiagnose vertical growth phase) should be added to the recommendations for melanoma histologic report, at least for level II SSM.