(20S) esters of camptothecin analogs are provided. The compounds are (20S) esters of an oxyalkanoic acid and camptothecin, which .[.is.]. .Iadd.are .Iaddend.optionally substituted at the 7, 9, 10, 11, and 12 positions of the camptothecin ring. The compounds are useful for treating cancer.