This papers deals with the determination of the influence of water and landfill leachate dosage on the quality and quantity of municipal waste biogas. Wastes used in the experiment were collected from a municipal landfill located in Łęgajny near Olsztyn. Gas production (total gas volume, production rates and methane concentration) was monitored for 311 days. The quality and quantity of biogas were determined as for waste deposition without irrigation and with irrigation with the addition of water and leachate (the amount was typical for the average atmospheric precipitation and double amount of atmospheric precipitation). Moreover, the researches were carried out for totally flooded wastes. It was shown that there is an influence of water conditions on the biogas production. The lowest biogas production was observed for waste deposition without water and leachate irrigation (dry wastes) and for totally flooded wastes. Obtained biogas characterized the lowest amount of methane. The highest effectiveness of biogas production and its best composition was achieved for wastes irrigated by leachate in the amount corresponded to atmospheric precipitation. Biogas contained about 50% of methane. Research showed that there is the possibility of biochemical processes controlling in waste heap (e.g. in energy piles); however, there is then the necessity of optimization of the dosage and kind of recirculated medium.