The invention relates to a method for transmitting packet switched data in a mobile communications system using an ARQ protocol. In the method the receiver orders the desired transmission units. The transmitter sends transmission units according to the order. Ordering and transmission are repeated until the quality measured by the receiver from each packet exceeds the predetermined quality value and the data which is to be transmitted and consists of packets is transmitted from the transmitter to the receiver. The invention also relates to a mobile communications system implementing the method of the invention.