For wireless multipath fading channels, the performance of a direct-sequence code-division multiple-access (DS-CDMA) system is degraded by the interferences, including the multipath interference (MPI) and the multiuser interference (MUI). For the downlink DS-CDMA with random spreading, we derive the variances of the interferences and obtain the signal-to-interference-plus-noise ratio (SINR). The simple and elegant expressions for the variances of the interferences provide useful insights, while the corresponding SINR representation is helpful in evaluating the bit-error-rate (BER) performance that takes the impact of the interferences into consideration. In order to demonstrate our approach, an example regarding the performance analysis of the two-input–single-output (2ISO) space–time block coded DS-CDMA with RAKE reception is also provided.