Cast　cobalt-chromium　(Co-Cr)　alloy　clasps　have　been　widely　used　for　prosthodontic　treatment,　but　long-term　retentiveness　of　these　clasps　is　not　clear.　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.　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　0.50　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　(α=.05)　to　compare　the　retentive　force　of　each　group　of　clasps　for　different　periods　of　fatigue　testing.　The　ANOVA.　The　Student　t　test　(α=.05)　was　used　for　analysis　of　clasps　fabricated　with　the　same　alloy　but　with　different　undercuts.　A　linear　correlation　analysis　(α=.05)　was　used　to　test　the　correlation　between　retentive　forces　and　cycling　sequences.　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).　A　negative　correlation　was　found　between　the　retentive　force　of　cast　Co-Cr　alloy　clasps　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.　©　2010　The　Editorial　Council　of　the　Journal　of　Prosthetic　Dentistry.