The therapeutic and prognostic evaluation of malignant neoplasias depends largely upon a precise morphologic diagnosis. Several papers have focused on the importance of fine-needle aspiration under computed tomography guidance in the diagnosis of unresectable neoplasms and in the investigation of metastases. The objective of this study is to evaluate the diagnostic accuracy, the sensitivity, and the negative predictive value obtained with the technique. Fine-needle aspiration cytology (FNAC) was performed on 207 patients, with a total of 210 cases, from 1991 to 1994, under computed tomography (CT) guidance. There were 128 (61.8 percent) males and 79 (38.2 percent) females with a mean age of 41 years (range 1 to 91 years). Lung and liver were the most frequent anatomic sites. The analysis of this material disclosed 41 cases with cytological diagnosis of negative for malignancy (19.52 percent), and in 131 (62.38 percent), the diagnosis was positive. It was possible to define the cytologic lineage in 54 percent of the cases. The diagnosis in 14 (6.67 percent) cases was suspicious for malignancy, and in 24 (11.43 percent) cases the material was insufficient for the cytologic diagnosis. Of the 210 cases, 106 showed histological diagnosis and/or clinical follow-up. Forty-seven (44.3 percent) had histological diagnosis before the FNAC and 50 cases (56.7 percent) histological diagnosis after the procedure. The comparison between cytological and histological diagnosis showed a sensibility of 80.4 percent, specificity of 100 percent, positive predictive value of 100 percent and negative of 16.7 percent. The efficiency of the test was 81.1 percent. This study showed that FNAC, under computed tomography guidance, is a sensitive and specific technique for the diagnosis of deep-seated lesions.