In data transmission and storage systems, combined error correction and detection procedures are often used to provide high reliability. This paper considers the use of separate concatenated codes C and D for error correction and detection, respectively. It examines the error detection performance of code D to determine how the probability of undetected error depends on the choice of C and D . A comparison is made of the probability of undetected error achievable by code D with and without error correction, respectively. Asymptotic bounds for low bit error rates are developed which provide criteria to be considered when choosing codes C and D .