A modification of the cell block technique, useful in processing material obtained by fine needle aspiration (FNA), is described. Four hundred six aspirates, obtained from 333 consecutive patients, were studied after immediate fixation in 4% buffered paraformaldehyde. Conventional histochemical and immunohistochemical staining methods were used. Histologic verification of the cytologic diagnoses made by FNA was possible in 67 cases. The overall accuracy was 97%, with a sensitivity of 95% and specificity of 100%. A major disadvantage of the cell block technique is time. Therefore, even if this technique increases the accuracy of cytologic diagnosis, its routine use is impractical because the delay in diagnosis when compared with smears may be considerable. The cell block technique is a valuable method, particularly when immunohistochemical staining for a battery of markers is required.