The Collaborative International Dictionary of English v.0.48:

obsessional \obsessional\ obsessive \obsessive\adj.
   Persistently and abnormally preoccupied with some
   unreasonable idea.

   Syn: compulsive.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

obsessive
    adj 1: characterized by or constituting an obsession; "the
           obsessional character of his response"; "obsessive
           gambling" [syn: obsessional, obsessive]
    n 1: a person who has obsessions

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "obsessive":
   absorbing, all-encompassing, arresting, attractive, compelling,
   compulsive, consuming, controlling, dominating, driving,
   enchanting, engaging, engrossing, enthralling, fascinating,
   gripping, harassing, haunting, holding, hypnotic, impelling,
   indelible, magnetic, mesmeric, mesmerizing, nagging, obsessing,
   obsessional, passionate, persistent, possessing, preoccupying,
   recurrent, spellbinding, tormenting, unforgettable, unshakable,
   unshakeable