1. 
[syn: grievous, heartbreaking, heartrending]
The Collaborative International Dictionary of English v.0.48:
Heartbreaking \Heart"break`ing\, a.
   Causing overpowering sorrow.
   [1913 Webster]
WordNet (r) 3.0 (2006):
heartbreaking
    adj 1: causing or marked by grief or anguish; "a grievous loss";
           "a grievous cry"; "her sigh was heartbreaking"; "the
           heartrending words of Rabin's granddaughter" [syn:
           grievous, heartbreaking, heartrending]
Moby Thesaurus II by Grady Ward, 1.0:
20 Moby Thesaurus words for "heartbreaking":
   afflictive, agonizing, calamitous, consuming, desolating, dire,
   distressing, excruciating, grievous, harrowing, heartrending,
   heartsickening, heartwounding, lamentable, racking, regrettable,
   rending, tormenting, torturous, unfortunate