The Collaborative International Dictionary of English v.0.48:

Retrogressive \Re`tro*gres"sive\, a. [Cf. F. r['e]trogressif.]
   1. Tending to retrograde; going or moving backward; declining
      from a better to a worse state.
      [1913 Webster]

   2. (Biol.) Passing from a higher to a lower condition;
      declining from a more perfect state of organization;
      regressive.
      [1913 Webster]

WordNet (r) 3.0 (2006):

retrogressive
    adj 1: going from better to worse [syn: retrograde,
           retrogressive]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "retrogressive":
   ascending, atavistic, axial, back, back-flowing, backward,
   coming apart, cracking, crumbling, decadent, declining, degenerate,
   descending, deteriorating, disintegrating, down-trending, downward,
   draining, drifting, drooping, dwindling, ebbing, effete, fading,
   failing, falling, flagging, flowing, fluent, flying, fragmenting,
   going, going to pieces, gyrational, gyratory, languishing,
   marcescent, mounting, passing, pining, plunging, progressive,
   reactionary, recessive, recidivist, recidivous, reflowing,
   refluent, regressive, retroactive, retrocessive, retrograde,
   retrorse, retroverse, returnable, reversible, reversional,
   reversionary, revertible, revulsionary, rising, rotary, rotational,
   rotatory, running, rushing, shriveling, sideward, sinking, sliding,
   slipping, slumping, soaring, streaming, subsiding, tabetic,
   up-trending, upward, waning, wasting, wilting, withering,
   worsening