A series of continuous vibration measurements in 14 upwind wind turbines of the same model and belonging to the same wind farm have been conducted. The data were acquired over a period lasting approximately half a year. The tower axial vibration acceleration has been monitored in the frequency band from 0 to 10 Hz with an accelerometer mounted on the gearbox casing between the intermediate and the high-speed shafts. It has been observed that the average frequency spectrum is dominated by the blade passing frequency in all the wind turbines. The evolution of the vibration magnitudes over the entire range of operating conditions is also very similar for all the wind turbines. The root-mean-square (rms) acceleration value has been correlated with the wind speed, and it has been found that a linear fit with a positive slope is a useful model for prediction purposes.