No study has evaluated the antiproliferative effects of thiazolidinediones and antiblastics in 'primary cultured human anaplastic thyroid cancer cells'. Primary anaplastic cells proliferation was evaluated after incubation with increasing concentrations of rosiglitazone or pioglitazone or antiblastics (bleomycin, cisplatin, gemcitabine) by a proliferation assay (WST-1-tetrazolium reaction) and cell counting. A reduction of proliferation by thiazolidinediones at 1 h (from the start of tetrazolium reaction) [of 11% and 25%, with rosiglitazone, 10 or 20 (P = 0.0001) mu M, respectively; of 7% and 17%, with pioglitazone, 10 or 20 (P = 0.0125) mu M, respectively], and at 2 h [of 14% and 24%, with rosiglitazone, 10 (P = 0.0043) or 20 (P < 0.0001) mu M, respectively; of 9% and 21%, with pioglitazone, 10 (P = 0.0397) or 20 (P = 0.0001) mu M, respectively] was shown. No significant thiazolidinediones effect was observed in normal thyroid follicular cells. Bleomycin, cisplatin and gemcitabine significantly (P < 0.0001) inhibited (> 50%) anaplastic cells proliferation. Cell counting confirmed the above mentioned results. Inhibition of proliferation was similar in tumours with or without (V600E)BRAF mutation, both for thiazolidinediones and antiblastics. Thiazolidinediones exert an antiproliferative effect in primary cultured human anaplastic carcinoma cells in vitro, such as antiblastics.