The olive oil has a healthy image during its consumption due to its oleic acid content, which may prevent some human diseases. Ironically, by-products of olive mill production such as olive mill effluent (OME) and olive cake pose a serious environmental risk where it is produced. In this study, feasibility of using some agro-industrial residue streams such as cheese whey (CW) and laying hen litter (LHL) in order to enhance the methane production of OME was investigated. For this purpose, biochemical methane potential (BMP) assay was carried out for both raw OME alone and OME mixed with varying amount of other substrates such as LHL and CW in the serum bottles, respectively. Corresponding methane production values for various mixtures of the organic residue streams used in this study were determined. It was demonstrated that co-digestion of OME with LHL significantly enhanced the biodegradability of OME which was too low if it was digested alone. Over 90% increase in biogas production was obtained when digesting OME with LHL. The biogas production increased only 22%, when CW was used for the same purpose. It was demonstrated that the biodegradability of OME could be significantly enhanced by co-digestion and thereby integrated management of OME using anaerobic degradation could be proposed as an economically viable and ecologically acceptable solution for the safe disposal of OME. (C) 2008 Elsevier Ltd. All rights reserved.