Statement　of　problem　Cast　cobalt-chromium　(Co　Cr)　alloy　clasps　have　been　widely　used　for　prosthodontic　treatment,　but　long-term　retentiveness　of　these　clasps　is　not　clear　Purpose　The　purpose　of　this　study　was　to　investigate　the　residual　retentive　force　of　3　types　of　cast　Co　Cr　alloy　clasps　in　relation　to　their　proportional　limits　Material　and　methods　Thirty　half-round　straight　clasps　and　30　circumferential　clasp　assemblies　were　made　from　3　Co-Cr　alloys　(Hardalloy,　Regalloy,　and　Vera　PDN)　(n=10)　All　of　the　straight　clasps　were　subjected　to　a　compression-deformation　test　to　obtain　data　for　displacement/load　at　the　proportional　limit　Two　undercut　depths　(0　25　mm　and　0　SO　mm,　n=5)　were　included　in　a　cyclic　fatigue　test　Each　circumferential　clasp　assembly　was　subjected　to　an　insertion/removal　test　on　a　metal　abutment　tooth　for　7200　cycles　to　simulate　5　years　of　clinical　use　The　residual　retentive　force　was　measured　after　360　cycles,　thus,　the　retentive　force　of　each　clasp　was　recorded　21　times　throughout　the　test　The　data　were　subjected　to　repeated-measures　ANOVA　(alpha　=　05)　to　compare　the　retentive　force　of　each　group　of　clasps　for　different　periods　of　fatigue　testing　The　ANOVA　The　Student　t　test　(alpha　=　05)　was　used　for　analysis　of　clasps　fabricated　with　the　same　alloy　but　with　different　undercuts　A　linear　correlation　analysis　(alpha　=　05)　was　used　to　test　the　correlation　between　retentive　forces　and　cycling　sequences　Results　The　mean　values　of　displacement　at　the　proportional　limit　for　the　3　Co　Cr　alloys　varied　from　0　30　mm　to　0　36　mm　The　mean　retentive　forces　of　each　alloy　group　decreased　dramatically　after　the　first　cycling　phase　(P　＜　05)　Thereafter;　the　decrease　was　more　gradual　but　was　still　significantly　different　compared　to　the　original　retentive　forces　(P　＜　05)　Compared　to　clasps　in　the　0　25-mm　undercut　group,　those　in　the　0　50-mm　undercut　group　exhibited　greater　mean　retentive　forces　before　the　cyclic　fatigue　test　(P　＜　05),　as　well　as　a　greater　decrease　in　the　retentive　force　at　the　end　of　the　test　(P　＜　05)　Conclusions　A　negative　correlation　was　found　between　the　retentive　force　of　cast　Co-Cr　alloy　clasp,　and　the　logarithm　of　cycling　sequences　After　a　test　simulating　5　years　of　service,　cast　Co-Cr　alloy　clasps　exhibited　a　residual　retentive　force　to　satisfy　the　requirements　for　clinical　use　(J　Prosthet　Dent　2010,104　389-396)