In cost-effectiveness analysis and contemporary treatment planning str
ategies, the postponement of placement of cast crowns plays an importa
nt role. Extensive amalgam restorations that involve the rebuilding of
cusps and the provision of auxiliary retention are thought to make th
is postponement possible. This study reports the long-term survival (1
00 months) of extensive amalgam restorations in a randomized, controll
ed clinical trial. The operational hypothesis was that the type of ret
ention and the operator have a long-term influence on the survival and
clinical functioning of extensive amalgam restorations. Three hundred
extensive amalgam restorations were placed by three operators in mola
r teeth in which one or more cusps were reconstructed. Five different
auxiliary retention methods were used for retention of these restorati
ons. Since the purpose of extensive amalgam restorations is considered
to be two-fold (to restore a broken-down molar to function acceptably
as an independent restoration and to create a substructure for subseq
uent crown construction), survival was assessed at different levels. T
he survival rate of extensive amalgam restorations as an independent r
estoration was 88 +/- 2%. The functional survival rate (as an independ
ent restoration or as a substructure) was 92 +/- 2%. The influence of
experimental variables ('retention method' and 'operator') and backgro
und variables ('tooth type', 'extension of extensive amalgam restorati
ons', and 'age of patient') on the survival was analyzed by Log Rank a
nd Breslow tests. The analyses revealed that there were no statistical
ly significant influences on the survival rates except for the variabl
e 'age of patient' (p less than or equal to 0.05). Extensive amalgam r
estorations were more prone to failure in the group of older patients
than in the group of younger ones. It is concluded that the clinical s
urvival of extensive amalgam restorations is independent from several
clinical variables in the study.