The invention relates to a data transmission method and a radio system that comprises a first (260) transceiver and a second transceiver (264) that are in radio contact with each other. The first transceiver (260) comprises means (500, 504) for forming data blocks for transmission in such a manner that the data blocks are given identifiers for identification. The second transceiver (264) comprises means (512) for receiving data blocks transmitted and retransmitted by the first transceiver, and means (524) for detecting a failure in receiving a data block. The second transceiver (264) comprises means (530) for maintaining information on the position of a window belonging to a finite identifier space, and means (524) for comparing the identifiers of the re-received data block and the earlier received data block with each other and for defining the data blocks as the same, and means (524) for combining the data blocks defined as the same.