A group of 434 patients treated for primary cutaneous melanoma over an 18-year period by a policy of selective excision margins based on clinical estimation of tumour thickness was studied prospectively. Clinical assessments of tumour as impalpable, palpable but not overtly nodular, and nodular correspond to thicknesses of < or = 0.75, 0.76-1.49 and > or = 1.50 mm respectively. From 1971 to 1987, 330 patients were treated with excision margins of 1, 2 and 3-5 cm based on these respective ranges. During 1988 and 1989, 104 patients were treated after reduction of the maximum width of excision to 2 cm. The overall local recurrence rate was 11 of 434 patients (2.5 per cent), with no recurrence in melanoma < or = 0.75 mm thick and no increase in the local recurrence rate after reduction of the maximum margin from 3 to 2 cm. A policy of 2-cm clearance for palpable and nodular melanoma and 1 cm for impalpable lesions is recommended. Any further reduction must be tested prospectively.