A 5(S)-(2'-hydroxyethoxy)-20(S)-camptothecin diasterisomer is described which is a better inhibitor of topoisomerase I than either the diastereoisomeric mixture 5(RS)-(2'-hydroxyethoxy)-20(S)-camptothecin, or the 5(R)-(2'-hydroxyethoxy)-20(S)-camptothecin diastereoisomer. Pharmaceutical compositions of the 5(S)-(2'-hydroxyethoxy)-20(S)-camptothecin diastereoisomer are also described as are methods of using the compound for the inhibition of topoisomerase I and for the treatment of cancer.