They are not true lilies but are mildly toxic. From this site:
http://www.aspca.org/site/PageServer...ter_friendly=1
"Peace lilies (Spathiphyllum spp.), and calla lilies (Zantedeschia spp) contain insoluble calcium oxalate crystals and should not be confused with those in the Liliaceae family; these plants will cause gastrointestinal upset and oral discomfort but do not cause renal injury if ingested by cats. Lily of the Incas or Peruvian lily (Alstroemeria aurantiaca) is also not a true lily, and is only expected to cause mild gastrointestinal signs in healthy animals. Another “faux-lily”, lily of the valley (Convallaria majalis) contains cardiac glycosides that can cause fatal arrhythmias similar to those seen in digoxin toxicosis."