Since dewatering equipment is commonly operated only during normal working hours, activated sludge must often be stored in an anaerobic condition prior to conditioning. It is the objective of this study to investigate the influence of anaerobic storage on conditioning requirements and dewatering performance on a laboratory scale. Sludges were collected at two large treatment plants (removal of organic matter, nitrogen and phosphorus) and one small treatment plant (removal of organic matter). Thickened activated sludges from the three wastewater treatment plants were stored anaerobically in the laboratory and analyzed frequently during fourteen days of storage. Both organic and inorganic conditioning was used. Turbidity and Dissolved Organic Carbon (DOC) in the sludge bulk water increased as a result of the anaerobic storage. These parameters indicated a release of colloids, dissolved exopolymers and fermentation waste products such as fatty acids to the bulk water. These constituents consumed additional cationic polyelectrolyte. Filterability at the optimal dosage of polyelectrolyte was not affected by anaerobic storage. Therefore, polyelectrolyte requirements are governed by the bulk water constituents, whereas filterability of the sludge is determined by the degree of sludge floc conditioning. Iron requirements seemed unaffected by anaerobic storage, but lime requirements to obtain good filterability increased with anaerobic storage time.