Abstract A general Brueckner-type theory of electron correlation, including double and triple substitutions, is presented (BDT theory). This has the properties of (1) exactitude for three electrons and (2) size consistency. A Møller-Plesset type of perturbation analysis shows that this is identical in fifth order to coupled cluster theory with single, double and triple substitutions based on Hartree-Fock (CCSDT). The BDT equations, however, are considerably simpler than those of CCSDT. Some preliminary applications of BDT theory are discussed.