A sensor ear tag (SET) containing Global Positioning System (GPS), accelerometer, Radio-Frequency Identification (RFID), and Bluetooth technologies was tested for wearing comfort and compliance with animal welfare requirements in cattle in a free stall barn and on summer pasture in Switzerland. The SET was equipped with a long-lasting battery via solar panel and used a «twin pin» fixing system. Right ears of 12 newborns and 26 adolescent animals were tagged with the SET. While left ears were tagged simultaneously with official ear tags in newborns, the adolescent animals already carried the official ear tags. The newborns stayed in a free stall barn during the entire experiment, while adolescent animals were housed in a free stall barn and on pasture during summer. All animals developed crusts beginning on day 7 after tagging with the SET. Pain reactions were observed occasionally in the first two weeks. Ear growth in newborns during 11 months of observation did not differ between ears with SET and official ear tags. Cortisol concentration in saliva of newborns decreased in the first week after tagging which is physiological for this age group. In older animals cortisol concentrations in saliva were not affected. We registered 19 incidences in 11 animals with the SET, that required veterinary or staff intervention. Two animals lost the SET with ear injury. Scars due to tag migration were observed in ears of all newborns after the 9th month of observation. In conclusion, SET with a weight of 32 g that need a twin pin fixation in cows do not seem to induce systemic or local inflammations more frequently compared to official ear tags; however, the higher risk of accidental injuries and migration in ear cartilage would not meet Swiss welfare standards and the attachment to the ear needs to be improved for general use.