This research explores the appropriateness of unbalancing workloads in flexible manufacturing systems (FMSs) configured as pooled machines of unequal sizes. Studies are conducted to examine the applicability of following an objective to unbalance workloads when solving the FMS planning problems of selecting part types to be machined together and determining their production ratios. Simulation is used to compare unbalancing and balancing on realistic, detailed models of flexible flow systems (FFSs). The experiments are constructed to evaluate the impact of such factors as blocking, transportation, buffer utilizations, fixture requirements of various types, and different workload distributions among the machine types. The results indicate that the aggregate and theoretically optimal unbalanced workloads provided by Stecke and Solberg [1981, 1985] using a closed queueing network model are appropriate in a realistic FMS. It is demonstrated that the production rate and system and machine utilizations are all higher when unbalancing workloads in systems of pooled machines of unequal sizes. It is also observed that unbalanced part mix ratios conversely lead to balanced machine utilizations among unequally pooled machine types. However, overall system utilization seems to be more sensitive to the number of pallets in the system when unbalancing than when balancing. Further research needs are also noted.