Disclosed herein polynucleotides complementary to IL-34, including IL-34 antisense oligonucleotides and IL-34 NAs, and methods for treating inflammatory diseases, such as an inflammatory bowel disease, and/or fibrosis, associated with elevated activity or expression of IL-34. Also disclosed are pharmaceutical compositions containing a polynucleotide complementary to IL-34, for example, an IL-34 antisense oligonucleotide or an IL-34 siRNA, useful for treating inflammatory diseases and/or fibrosis and manufacture of medicaments containing a disclosed polynucleotide to be used in treating inflammatory diseases and/or fibrosis.