Of the anthropogenic greenhouse gas emission in Denmark animal manure contributes an estimated 40% of methane (CH4) and 20% of nitrous oxide (N2O). Livestock production systems undergo changes for the purpose of increasing animal welfare, and such changes often include increasing the amounts of bedding manure. Emission of greenhouse gases from composting pig deep litter was studied during a 4-month period. Effects of increasing the amount of straw used in deep litter (reducing litter density) were included in the study. Methane was produced at a high rate in the centre of the heap at high density during the thermophilic phase of composting, and CH4 emission was only measured during this phase. In this treatment N2O was also produced in the centre both initially and after the temperature of the compost had dropped to below 45 °C. Emissions of N2O were only significant in the low temperature phases. Production of N2O was probably restricted to the surface layers during the thermophilic phase of composting. Total carbon dioxide (CO2) emissions were 7·37 and 0·09 kg C/t fresh weight from the heaps with bulk densities of 0·44 and 0·23 kg/l, respectively. Methane emission from the high density compost heap was 191 g C/t and N2O emission was 58 g N/t. Emissions of CH4 and N2O from the low density heap were not detected. The greenhouse effect of gas emission from the high density compost heap was calculated by multiplying the climate force efficiencies and the cumulated gas emission. This calculation showed that CH4 contributes almost as much to the global warming potential as CO2, and N2O contributed twice as much as CO2.