Prostaglandin E.sub.1 ester derivatives of the formula (I): ##STR1## wherein the R' groups are the same as each other and are C4-12 alkyl; or cyclodextrin clathrates thereof, liposome formulations containing them, and pharmaceutical compositions containing them, as active ingredient. The compounds of formula (I) have the effect of increasing blood flow and are useful for the prevention and/or treatment of peripheral circulatory disorder, decubitus, or skin ulcers, or for the maintenance of blood flow.