Indoor nitrogen dioxide exposure has been associated with respiratory symptoms in children in many studies, but in Australia, levels and sources of nitrogen dioxide in homes have not been well-characterized. Therefore, as part of a larger indoor environmental study, conducted in the Latrobe Valley, Victoria, nitrogen dioxide was monitored using passive samplers in 80 homes. Samples were collected on five occasions over one year. Mean indoor levels were higher than outdoor levels, and a seasonal variation was evident, with highest levels recorded in winter. The overall median level was 11.6 micrograms/m3 (6.0 ppb), ranging from < 0.7 to 246 micrograms/m3 (128 ppb). Major indoor nitrogen dioxide sources were: gas stoves, vented gas heaters, and smoking. Some 67% of variation in indoor nitrogen dioxide levels could be explained by presence of major sources, house age, and outdoor levels. Gas stoves were the main contributors.