Our aim was to identify the optimal diagnostic cutoff point on the scale of protrusion measurements of the first metatarsal (M1) to predict the probability of reulceration after metatarsal head resection in patients with diabetes mellitus. We conducted a prospective study of patients with diabetes who underwent resection of at least 1 metatarsal head in our department. After surgery, we measured the difference in length (protrusion) between the M1 and the longest of the 4 lesser metatarsals by radiographic view. The patients were divided into those in whom the M1 was the longest of the 5 metatarsals (group 1) and patients in whom at least one of the lesser metatarsals was longer than the M1 (group 2). They were followed-up for 12 months and were assessed for reulceration. Ninety-one patients were included in the present study: 43 (47%) in group 1 and 48 (53%) in group 2. In group 1, the longer the protrusion of M1 was, the higher the probability for reulceration (P < .001, 95% confidence interval = 0.813-0.997). In group 2, the shorter the protrusion of M1, the higher the probability for reulceration (P = .002, 95% confidence interval = 0.628-0.905). The optimal cutoff point for group 1 was 11 mm (sensitivity = 84.6%, specificity = 86.7%) for the probability of reulceration. In group 2, it was -7 mm (sensitivity = 81.8%, specificity = 65.4%). These results suggest that M1 protrusion is an optimum prognostic indicator for reulceration and could be recommended for detecting patients at risk of reulceration after surgery.