Whey, produced in large quantities during cheese production, is a rapidly fermentable high strength wastewater characterized by a high biodegradability and low alkalinity. In this study, a lab-scale cross-flow anaerobic membrane bioreactor was used to address the commonly experienced difficulties such as unstable reactor performance and unexpected biomass losses when treating whey wastewater with conventional anaerobic reactors. The anaerobic membrane bioreactor provided a stable treatment performance, i.e. more than 90% chemical oxygen demand removal, and moderate membrane fluxes between 8 and 11 L m(-2) h(-1) could be obtained, applying a low cross-flow velocity of about 0.5 m s(-1). Short term critical flux tests revealed that higher fluxes up to 36 L m(-2) h(-1) are possible at elevated cross-flow velocities and/or reduced mixed liquor suspended solids concentrations. Sludge filterability indicated by capillary suction time and specific resistance to filtration deteriorated throughout the study. Chemical cleaning efficiency gradually decreased, indicating irreversible membrane fouling during long term operation.