Changes of the toroidal rotation velocity of up to 3x104 m/s have been observed during ICRH in JET. The plasma increases its toroidal rotation in the direction of the plasma current. The change in velocity is obtained from the measurement of the Doppler shifted X-ray lines emitted from He-like nickel present in the centre of the plasma. By analysing various heating scenarios and also measuring the fast anisotropic ion energy content of the plasma we conclude the rotation is connected with the creation of fast ions during ion cyclotron resonance heating. Toroidal acceleration of the plasma in connection with MHD-instabilities leading to similar magnitude of rotational velocity is also seen.