PURPOSE: To determine the rate of successful biopsy and frequency of post-biopsy pneumothorax and hemorrhage using an 18-gauge cutting needle in CT-guided automated needle biopsies according to lesion size, and the distance between lesion and pleura. MATERIALS AND METHODS: Ninety-four patients with focal lung lesion who had undergone CT-guided automated needle biopsies using an 18-gauge cutting needle were retrospectively reviewed. We evaluated the relationship between successful biopsy rate and pneumothorax and hemorrhage according to lesion size and distance between lesion and pleura. For the purposes of this study, size and distance were grouped as follows : < or = 1cm ; 1 . 1 -2cm; 2.1 -3cm ; >3cm . RESULTS: Biopsy was successful in 78 of 94 patients(83%). When lesions were larger than 2cm in size the diagnostic rate increased (P=0.002), but the distance between lung lesion and pleura was not statistically related to successful biopsy (P>0.005). Where there were post-biopsy complications, the pneumothorax rate was higher in lesions less than 2 cm in size (P=0.041) and in those separated by more than 2 cm from the pleura (P= 0.006). Where separation was of this order, the hemorrhage rate was higher (P=0.021), but statistically, this was not affected by lesion size (P>0.05). CONCLUSION: When 18-gauge cutting needle is used in CT-guided automated needle biopsies of pulmonary lesions, the rate of successful biopsy is affected by lesion size ; the pneumothorax rate is also affected by lesion size, as well as by the distance between lesion and pleura; the hemorrhage rate is influenced only by the distance between lesion and pleura.