From 1973-1989 34 patients with poorly differentiated primarily inoperable and 89 with anaplastic giant-cell thyroid tumours entered a study on individualized multimodal treatment. The aim of the study was to find an effective, non-aggressive chemotherapy (ChT) and explore the possibility of individualized combined treatment. Infusions of low doses of vinblastine or cisplatinum or adriamycin or novantrone were used for perturbation of cellular kinetics in the tumours. Sequential fine-needle aspiration biopsies of tumours were performed for monitoring the drug-induced changes in tumour cells and for cytomorphological studies and DNA measurements, on the basis of which ChT was individualized. During the period of accumulation of cells in the S or G2 + M phases, drugs particularly effective in the respective phases were used. The same principle was used in planning a combination of ChT and irradiation. In the group of poorly differentiated carcinomas only 5/34 patients had MR to ChT. In 17/34 patients after ChT the tumour was removed surgically, 19/34 have NED 11+ to 72+ months after treatment. The results in 89 patients with anaplastic carcinoma are worse: 13/89 patients died before or at the beginning of treatment, of 77 treated patients only 10 could be operated on, 7 survived for more than one year (13+ to 72+ months). Only 1/77 patients survived for more than 5 years. A combination of DNA measurements and cytomorphological studies is a useful method for the understanding of tumour behaviour under treatment.