A precise bit error rate analysis is performed for bandwidth-efficient dual-channel quadrature phase shift keying in Nakagami fading with Nakagami-faded asynchronous interfering signals. In contrast to previous works, the analysis accounts for the dependencies between the quadrature carriers owing to non-Rayleigh fading and cochannel interference. The impact of the interfering users' fading severities on the desired user bit error rate is investigated. A comparison of the bit error performance for binary phase shift keying and quadrature phase shift keying is given.