In previous studies we found high concentrations of the neurotransmitter glutamate in chronic painful tendons. To evaluate the possible importance, the high intra-tendinous glutamate concentrations had for the pain suffered in chronic Achilles tendinosis, microdialysis was performed before and after treatment. The results showed that in patients that were pain-free after treatment there were no significant differences in the glutamate levels before compared to after treatment. With this finding in mind, also other possibly pain-related mechanisms were evaluated. Using ultrasonography and colour doppler technique, we found that in chronic painful tendinosis tendons, but not in normal pain-free tendons, there was a neovascularisation inside and outside the area with structural tendon changes and pain. To test the hypothesis that there was an association between neovascularisation and pain, in a pilot study, under ultrasound and colour doppler guidance the area with neovascularisation was destroyed by injecting the sclerosing agent Polidocanol. The clinical results showed that 8 / 10 patients were pain-free and had no remaining neovessels. The 2 patients that were not pain-free had remaining neovessels. In a following investigation combining ultrasonography + colour doppler, immunohistochemical analyses of biopsies, and diagnostic injections, the results showed that in the area with tendon changes and neovascularisation, biopsies showed nerve structures in close relation to the blood vessels, and injetions of local anaesthesia temporarily cured the pain in all patients. Althogether, the findings indicate that the area with neovascularisation (neovessels and nerves) might be responsible for the pain suffered in chronic Achilles tendinosis, and that a locally administrated (in the area with neovascularisation) sclerosing drug (Polidocanol) has the potential to cure the pain.