The objectives of the present study were to understand the impact that occupant behaviour has on the generation of ultrafine particles from wood-burning stoves and to quantify particle concentrations when households use different types of stoves. The study was carried out in five typical Danish detached houses. The study was conducted during the winter without other indoor activities taking place. Ultrafine particle concentrations were monitored by means of two condensation particle counters. A mass balance model was used to predict the strength of indoor sources. The results showed that the highest particle concentrations were between approximately 0.05·1011 p/m3 (background level), when there was no source strength, and approximately 2.30·1011 p/m3, for the highest source strength. The results also showed that the concentration of ultrafine particles during the active burning period influenced by the occupants' every day use of the stove.