The incidence of postoperative neurological symptoms after performing interscalene block varies between 4 and 16%. The majority of cases are resolved spontaneously within a year, but some patients have their symptoms permanently. Our objective was to assess the incidence of postoperative neurological symptoms after performing the ultrasound-assisted interscalene and supraclavicular anaesthetic blocks. A prospective and observational study was conducted on consecutive patients who had undergone upper extremity surgery with an interscalene or supraclavicular block as an isolated technique, or as a complement to general anaesthesia. Seven days after the intervention, a telephone interview was conducted that focused on the detection of neurological symptoms in the operated limb. Further serial interviews were conducted on patients with symptoms (after the first, the third and the sixth month, and one year after surgery) until resolution of symptoms. Neurological evaluation was offered to those patients with persistent symptoms after one year. A total of 121 patients were included, on whom 96 interscalene blocks and 22 supraclavicular blocks were performed. Postoperative neurological symptoms were detected in 9.9% (95% CI, 5-15%) of patients during the first week. No significant differences were observed between interscalene (9%) and supraclavicular block (14%). After 3 months the symptoms persisted in 9 patients (7.4%), with symptoms remaining in 4 patients (3.3%) after 1.5 years. Electromyogram was performed on 3 patients who tested positive for nerve damage. A high incidence of postoperative neurological symptoms was observed, and a worrying percentage of permanence of them. There were no significant differences in incidence according to the type of block, or any features of the patient or the anaesthesia technique that were associated with the incidence of these symptoms, except a marginal relationship with age. These complications must be clearly explained to the patients before performing these blocks.