The Collaborative International Dictionary of English v.0.48:

Unrewarding \Unrewarding\
   See rewarding.

WordNet (r) 3.0 (2006):

unrewarding
    adj 1: not rewarding; not providing personal satisfaction [ant:
           rewarding]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "unrewarding":
   abortive, barren, bootless, deleterious, detrimental,
   disadvantageous, disserviceable, fruitless, gainless, harmful,
   injurious, nonremunerative, otiose, prejudicial, profitless,
   rewardless, sterile, unadvantageous, unfavorable, unproductive,
   unprofitable, unremunerative, useless, worthless