Disturbed vascularity leads to impaired fracture healing. Since low-intensity pulsed ultrasound (LIPUS) increases new bone formation in delayed-unions, we investigated whether LIPUS increases blood supply in delayed-unions of the osteotomized fibula, and if LIPUS-increased bone formation is correlated to increased blood supply. Blood vessel parameters were analysed using histology, immunohistochemistry, and histomorphometric analysis as well as their correlation with bone formation and resorption parameters. Fibular biopsies of thirteen patients with a delayed-union of the osteotomized fibula treated for 2-4 months with or without LIPUS originating from a randomized prospective double-blind placebo-controlled clinical trial were studied. In histological sections of the fibular biopsies parameters of blood vessel formation were measured and were related to histomorphometric bone characteristics of newly formed bone of the same samples analysed in our previously published study on the effects of LIPUS on bone healing at the tissue level in delayed-unions. LIPUS-treated delayed-unions and sham-treated delayed-unions as well as healed delayed-unions and failed-to-heal delayed-unions were compared. The volume density of blood vessels was increased in LIPUS-treated delayed-unions compared to sham-treated controls. LIPUS did not change blood vessel number, but significantly increased blood vessel size. Healed delayed-unions as well as LIPUS-treated and sham-treated delayed-unions showed significant correlations between blood vessel size and osteoid volume. LIPUS increases blood vessel size, essential for fracture healing, in bone from patients with a delayed-union of the osteotomized fibula. The increased osteoid volume in delayed-unions can largely be explained by increased blood supply and perfusion.