C.sub.2-C.sub.5-Alkyl-substituted [(imidazol-1-yl)-1-hydroxy-1-phosphono-ethyl]-phosphonic acids, as well as methods or processes for their manufacture, their use in the manufacture of pharmaceutical formulations, their use in the treatment of diseases, methods of using them in the treatment of diseases, pharmaceutical formulations encompassing them and/or the compounds for use in the treatment of diseases, are described. The compounds are able to inhibit excessive or inappropriate bone resorption and for the treatment of other diseases which are caused by excessive prenylation of target proteins, such as Hutchinson-Gilford progeria syndrome. The compounds are of the formula I, ##STR00001## wherein one of R.sub.1 and R.sub.2 is hydrogen and the other is C.sub.2-C.sub.5-alkyl that is branched or unbranched, and can be in free form, in the form of an ester, and/or of a salt.