Background: The aim of the study was to assess the influence of threshold density on automatic volume measurement of lung nodules, and to determine the range of optimal threshold density for automatic volume measurement. Material/Methods: The clinical material consisted of 28 patients with lung nodule. A chest CT examination was performed in each patient. After the examination, the nodule volume was assessed using Volume CT software, enabling both manual and automatic volume measurement. The volume of each lesion was measured both automatically and manually. The automatic measurement was performed with four different threshold densities: -250, -200, -150 and -100 HU. The results from automatic measurement were compared with those from manual measurement. Results: We found an obvious relation of the results of automatic volume measurement to threshold density. The higher the threshold density, the lower the value of automatic volume measurement. The agreement of manual and automatic measurements was significantly more common in the density range from -200 to -150 HU (57%), than for the range -250 to -200HU and for range -150 to -100HU (29% and 14% respectively). Conclusions: The results of automatic volume measurement depend directly on the chosen threshold density. The value of automatic volume measurement is lower for higher threshold density. The optimal threshold density range according to our study is from -200 to -150HU.