A new styryl lactone, 5 R,6 R-5-hydroxy-6-styryltetrahydropyrane-2-one 2 was isolated from the roots of an endemic Goniothalamus lanceolatus Miq. of Sarawak, Malaysia. Furthermore, seven previously undescribed diastereomers, 5 R,6 R-5-hydroxygoniothalamin 3, 5 R,6 R-5-acetylgoniothalamin 4, 6 S,7 S,8 S-goniodiol-7-monoacetate 5, 6 S,7 S,8 S-goniodiol-8-monoacetate 6, goniofupyrone B 7, deoxygoniopypyrone B 8 and 1 S,5 S,7 R,8 S,3- endo,7- endo-(+)-8- epi-9-deoxygoniopypyrone acetate 9, along with six known styryl lactones (1, 10–15) were also isolated and characterized. 6 S-goniothalamin 1 is reported for the first time from a Goniothalamus species. 1, 11 and 12 showed cytotoxic activity against human colon and lung cancer cell lines with IC₅₀ values ranging from 2.38–7.59 μM.