The purpose of the study was to find out whether electron microscopy (EM) was possible on tumour cells obtained by trans-thoracic fine needle aspiration biopsy (FNB). In a consecutive series of 105 FNB light microscopic (LM) evaluation of the aspirate demonstrated tumour cells in 67 cases. In 43/67 cases (64%) it was possible to make an ultrastructural diagnosis on part of the FNB. In none of the 43 cases did the EM and LM of FNB diagnosis disagree as to major tumour cell type. However, in 11/43 cases EM of the FNB was able to contribute to a more specific histogenetic morphological tumour diagnosis.