A series of 6-thioxopyrimidines (5, 6), their 6-oxo- analogs (11-14), and pyrimidine-2,4-diones (20-26), were synthesized and evaluated for their antitumoral activity against 60 tumoral cell lines. The activity of propenethioamide (3, 4) and propeneamide (7-10 and 15-19) intermediates is also reported. Among the tested compounds the thioxopyrimidine 5c, bearing an N'-benzyl group, showed the best cytostatic activity. Furthermore, high selectivity and cytotoxic activity on the HOP-92 cell line of non-small cell lung cancer was exhibited by 3-amino-2-[(methylamino)thioxamethyl]-3-pyrrolidino-2-propenenitrile (3a).