The objective of this study was to evaluate the level of healing of chronic neuropathic plantar ulcers, using an irremovable windowed fibreglass cast boot, which is only opened after healing. A single-centre prospective study of a cohort of 177 diabetic patients with chronic neuropathic plantar ulcers was carried out. The duration of neuropathic plantar ulcers was 604 ± 808 days, with a mean surface area of 4.6 ± 6.5 cm(2) , a mean depth of 1.04 ± 1.08 cm and a mean volume of 5.9 ± 17.7 cm(3) . After a mean of 96 days of wearing a windowed fibreglass cast boot (min 9 days, max 664 days and median 68 days), the level of healing reached 83.6%, although 29 patients did not heal (16.4%). The compliance was at 95%. NPUs with bigger volumes (p = 0.037) and those located at the heels ( p = 0.004) had significantly lower healing levels. Twenty-one patients had moderate peripheral arterial disease (12%), and 24 patients were ostectomized for underlying osteomyelitis (14%), before inclusion. Moderate peripheral arterial disease (p = 0.970) or operated osteomyelitis (p = 0.128) did not modify the level of healing significantly, which were of 81% and 70.8%, respectively. Complications include 12 ulcers due to the windowed fibreglass cast boot (i.e. 7%) and two other ulcers being moderately infected, resulting in 2% of toe amputation, but there was no major amputation or phlebitis. The treatment of old and deep NPUs of the diabetic foot by wearing a windowed fibreglass cast boot without opening the boot prior to healing offers very high ulcer recovery levels. Windowed fibreglass cast boots were changed in only 26 cases (14.6%). In addition, compliance was excellent and of the order of 95%. Furthermore, moderate peripheral arterial disease or a recent ostectomy did not affect the efficacy of windowed fibreglass cast boot.