281 patients managed for primary clinical Stage I invasive cutaneous malignant melanoma at one Plastic Surgery Unit were followed up to a minimum of 10 years after primary surgical treatment. Sixty-three (23%) had received an initial biopsy procedure prior to definitive wide margin excisional surgery. One third of all lesions initially treated by so called incisional biopsy were rendered histologically unassessable on current histopathological criteria. Incisional biopsy significantly interfered with the accurate histopathological staging of the tumours. Of the assessable incisional biopsy specimens the majority were greater than 4.0 mm thick. When the incidences of local tumour recurrence and mortality were related specifically to maximal tumour thickness of the primary lesion, prognosis at minimum follow up of 10 years was not significantly different between patients treated initially by either incisional biopsy, minimal margin excisional biopsy or primary wide excisional surgery. Whether or not incisional biopsy adversely affects prognosis in cutaneous malignant melanoma, the technique should be avoided since it compromises accurate histopathological microstaging which represents one of our most valuable prognostic guides. Clinical doubt over the diagnosis of cutaneous malignant melanoma should be resolved by a total excisional biopsy of the lesion in question.