BACKGROUND: In the transition from the sixth to the seventh edition of the American Joint Committee on Cancer (AJCC) melanoma staging system, mitotic activity was incorporated, while Clark level of invasion was abandoned. OBJECTIVES: To investigate the effect of this change on the pathological tumour (pT)1 substaging of primary cutaneous melanomas and the possible clinical implications. METHODS: Patients with pT1 melanomas, diagnosed in the period January 2003 to March 2011, were selected from a population-based cohort study on cutaneous melanoma in the eastern part of the Netherlands. The pT1 melanomas were systematically reviewed by an expert pathologist and classified according to both the sixth and the seventh editions of the AJCC staging system. The shift of melanomas between pT1 substages, classified according to the two staging systems, was determined. RESULTS: In total, 260 pT1 melanomas were included. Overall 28% (57/207) of all pT1a melanomas shifted to pT1b when classified according to the new seventh staging classification, because of the presence of mitoses. Some 32% (17/53) of all pT1b melanomas shifted to pT1a. The percentage of pT1b melanomas relative to all pT1 melanomas increased from 20% to 36%. CONCLUSIONS: The addition of mitotic activity to the pathological staging system, according to the seventh edition of the AJCC staging system, resulted in a considerable change in the classification of thin cutaneous melanomas. This shift has clear clinical implications, as it is advised in the Dutch guideline that patients with pT1b melanoma should be offered a sentinel lymph node biopsy.