Imidazole compounds having adenosine deaminase inhibitory activity represented by the formula (I) wherein R1 is aryl or heterocyclic group which is optionally substituted with substituent(s); R.sub.2 is lower alkyl; R.sub.3 is hydroxy or protected hydroxy; -A- is lower alkylene; and --X-- is --O-- or --S--; provided that when then R.sub.1 is aryl which is substituted with substituent(s), or heterocyclic group which is optionally substituted with substituent(s), its prodrug, or their salt. The compounds are useful for treating and/or preventing diseases for which adenosine is effective. ##STR00001##