The performance of an anaerobic sequencing batch reactor (ASBR) was assessed when submitted to increasing organic load with different influent concentrations and cycle lengths. The 5-L mechanically stirred (75 rpm) ASBR contained 2 L of granular biomass and treated 2 L of synthetic wastewater per cycle. Volumetric organic loads (VOLs) from 0.66 to 2.88 g of chemical oxygen demand (COD)/(L·d) were applied by using influent concentrations from 550 to 3600 mg of COD/L in 8- and 12-h cycles. Reactor stability was maintained for VOLs from 0.66 to 2.36 g of COD/(L·d), with organic matter removal efficiencies for filtered samples (εF) between 84 and 88%. For VOLs from 0.78 to 2.36 g of COD/(L·d) at an influent concentration of 2000 mg of COD/L, when cycle length was reduced from 12 to 8 h, εF did not vary, yet showed a very distinct behavior from the other conditions. In addition, two operation strategies were studied for VOLs with approximately similar values of 2.36 and 2.08 g of COD/(L·d). One involved operation with an influent concentration of 2000 mg of COD/L and an 8-h cycle, whereas the other involved an influent concentration of 2600 mg of COD/L and a 12-h cycle. Only the former resulted in system stability and efficiency. These results indicate that besides organic load, influent concentration and cycle length play a significant role in ASBR systems.