OBJECTIVE To design user friendly guides to prognosis for patients who have had invasive primary cutaneous malignant melanomas surgically excised. METHODS Adaptation of the classification tree method was used to derive prognostic trees for four different subgroups of malignant melanoma patients in whom known and possible prognostic variables interacted in different ways. METHODS Scotland. METHODS Statistical modelling for prognostic trees was based on 1978 patients whose primary malignant melanoma was first diagnosed in 1979-86 for whom five year follow up and all relevant clinical pathological data were available. The resultant model was validated with 300 patients first diagnosed in 1987 for whom the same information was available. METHODS Actual and predicted rate of survival after diagnosis of primary cutaneous malignant melanoma. RESULTS The four subgroups of patients were men and women with ulcerated and non-ulcerated cutaneous primary melanomas. Validation of the model showed excellent agreement between actual status of patients in the relevant subgroups and their status as predicted by the model. CONCLUSIONS The prognostic trees are simple to use and give more accurate prognosis for individual patients than is currently available from tumour thickness alone.