Few papers investigate synchronization error effects for the uplink of orthogonal frequency-division multiple access (OFDMA) systems. By contrast, this paper simultaneously analyzes the joint effects of major synchronization errors, including symbol time (ST) offset (STO), carrier frequency offset (CFO) and sampling clock frequency offset (SCFO) for the uplink of OFDMA systems in time-variant multipath fading channels. Such errors degrade the performance of an OFDMA receiver by introducing inter-carrier interference (ICI), intersymbol interference (ISI) and multiple-access interference (MAI) into the systems. A theoretical signal-to-interference-and-noise ratio (SINR) is formulated to characterize the losses due to synchronization errors in time-variant multipath fading channels. The results provide designers a useful reference in designing suitable synchronization algorithms for the OFDMA applications.