The Collaborative International Dictionary of English v.0.48:

Mournful \Mourn"ful\, a.
   Full of sorrow; expressing, or intended to express, sorrow;
   mourning; grieving; sad; also, causing sorrow; saddening;
   grievous; as, a mournful person; mournful looks, tones, loss.
   -- Mourn"ful*ly, adv. -- Mourn"ful*ness, n.
   [1913 Webster]

   Syn: Sorrowful; lugubrious; sad; doleful; heavy; afflictive;
        grievous; calamitous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

mournful
    adj 1: expressing sorrow [syn: mournful, plaintive]
    2: filled with or evoking sadness; "the child's doleful
       expression"; "stared with mournful eyes"; "mournful news"
       [syn: doleful, mournful]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "mournful":
   Jeremianic, affecting, afflicted, afflictive, aggrieved, anguished,
   bitter, bleak, blue, calamitous, careworn, catastrophic, cheerless,
   cinerary, comfortless, complaining, deplorable, depressing,
   depressive, desolate, despairing, despondent, dirgelike,
   disastrous, discomforting, disconsolate, disheartening, dismal,
   dismaying, dispirited, distressful, distressing, doleful,
   dolorific, dolorogenic, dolorous, downhearted, dreary,
   dumb with grief, epitaphic, exequial, faultfinding, feral, forlorn,
   fretful, funebrial, funebrious, funebrous, funeral, funerary,
   funereal, grief-stricken, griefful, grieved, grievous, heartbroken,
   heartsick, howling, in grief, inconsolable, joyless, lamentable,
   lamentive, lugubrious, melancholy, moanful, mortuary, moving,
   necrological, obituary, obsequial, overcome, painful, pathetic,
   peevish, petulant, piteous, pitiable, plaintive, plangent,
   plunged in grief, poignant, prostrate, puling, querulous,
   regrettable, rueful, sad, saddening, sepulchral, sharp, sore,
   sorrowed, sorrowful, sorrowing, tearful, touching, tragic, ululant,
   uncomfortable, unhappy, upsetting, wailful, whimpering, whining,
   whiny, woebegone, woeful, wretched