Evaluation of tumor progression by repeated fine needle biopsies in prostate adenocarcinoma: modal deoxyribonucleic acid value and cytological differentiation. Repeated fine needle aspiration biopsies of the prostate were taken during a period of 24 months or more from 84 patients with untreated prostate cancer. Serial followup regarding modal deoxyribonucleic acid values and cytological differentiation of the tumor cells was possible in 72 and 78 patients, respectively. During followup the modal deoxyribonucleic acid values in the tumor cells changed towards an increased aneuploidy in 17 patients and the cytological differentiation decreased in 18. These findings of a change in modal deoxyribonucleic acid values and cytological differentiation of prostate cancer cells during the course of untreated patients support the concept of a gradual dedifferentiation of prostate cancer. 