Percutaneous fine-needle biopsy was used to investigate thoracic and abdominal masses in the dog and cat. One hundred and thirty-two cases were included in the study; 20 cases were excluded from the comparative study due to poor cellularity or blood contamination (retrieval rate 86(.)8 per cent). One hundred samples (56 dogs and 44 cats) were classified by cytology as neoplastic. All the cytological diagnoses of neoplasia were confirmed by histological samples obtained either by non-surgical methods, at surgery or during postmortem examination. No false positive diagnoses of neoplasia were made. Thirty-two samples were cytologically classified as 'negative for neoplasia'. Subsequent histological examination revealed 18 true negative and 14 false negative results. The procedure had an overall 89(.)4 per cent (118 cases out of 132) agreement between the diagnosis of inflammatory disease versus neoplasia, with a sensitivity of 87(.)8 per cent, a specificity of 100 per cent, a predictive value of a positive test of 100 per cent and a predictive value of a negative test of 56(.)3 per cent.