The invention provides a method for detection of inflammatory disease in a subject that comprises assaying a test sample of peripheral blood from the subject for a marker of DNA damage. An elevated amount of marker present in the test sample compared to control sample is indicative of inflammatory disease activity, including sub-clinical inflammation. The method can be adapted for quantitatively monitoring the efficacy of treatment of inflammatory disease in a subject. Markers of DNA damage include single- and/or double-stranded breaks in leukocytes, oxidative DNA damage in leukocytes, or a marker of nitric oxide oxidative activity (protein nitrosylation in leukocytes). The inflammatory disease can be inflammatory bowel disease (ulcerative colitis or Crohn's disease). The invention may also be used for detection of other types of inflammatory disease, such as non-immune intestinal inflammatory disease (diverticulitis, pseudomembranous colitis), autoimmune diseases (rheumatoid arthritis, lupus, multiple sclerosis, psoriasis, uveitis, vasculitis), or non-immune lung diseases (asthma, chronic obstructive lung disease, and interstitial pneumonitis). This unexpected discovery of markers of genotoxicity present in circulating leukocytes enables detection of inflammation occurring at a localized site with a relatively simple and minimally invasive assay using peripheral blood.