['Thin melanomas have become increasingly prevalent, and lesions 1 mm or less in thickness are frequently diagnosed. They are considered highly curable when treated with wide local excision alone with reported 5-year disease free survivals of 95% to 98%. However, thin Clark level III and IV melanomas may have an increased potential for metastasizing and late recurrence because of dermal lymphatics located at the interface of the papillary and reticular dermis. We have addressed this controversial area by reviewing the outcomes of patients with invasive thin (< or = 1.0 mm thick) melanomas.', 'We reviewed 415 invasive melanomas from 1983-1995 in the Rhode Island tumor registries which kept records of both tumor thickness and Clark levels. Sixty-eight (16.4%) of the 415 invasive melanomas were thin (< or = 1.0 mm in thickness) and were treated by wide local excision only. In situ lesions were excluded. Thirty-eight (56%) of the 68 thin melanomas were either Clark level III or IV.', 'Seven (18.4%) of the 38 level III and IV thin melanomas had a recurrence at a minimum follow-up of 36 months. Median time to recurrence was 52 months, and the average measured depth of tumor thickness was 0.84 mm. Only one (3.3%) of 30 level II melanomas recurred (P < .05).'] Are thin < or = 1 mm level III and IV melanomas higher risk lesions for regional failure and warrant sentinel lymph node biopsy?