Successful co-composting of solid and liquid olive mill wastes (OMW) and obtaining a product of horticultural value may increase the viability of this recycling approach. Two composting cycles were performed, in which olive mill solid wastes (OMSW) were used to form five mixtures, wetted either with fresh water or with olive mill wastewater (OMWW). Up to similar to 0.3 m(3) of OMWW could be applied to each m3 of the raw materials without negatively affecting the chemical, physical and horticultural properties of the resulted composts. A growing media composed of perlite amended with 25-33% OMW-composts showed higher suppressiveness against Fusarium oxysporum f. sp. melonis as compared to equivalent perlite:peat moss mixtures. The yields of tomato plants grown in peat moss amended with 20% (v:v) of OMW-composts were not significantly different than plants grown in unamended peat. The viability of co-composting as a treatment approach for OMWW is discussed in the context of management aspects and the horticultural value of the final product. (C) 2010 Elsevier Ltd. All rights reserved.