Leukocytes are thought to play an important role in the pathogenesis of inflammatory bowel diseases; granulocyte–monocyte adsorptive (GMA) apheresis, an extracorporeal technique aimed at removing activated circulating leukocytes from the blood, may represent a safe and effective therapeutic tool in these patients. The Italian Registry of Therapeutic Apheresis performed an observational, multicentric study involving 24 Gastroenterology Units. In this study, laboratory data and clinical outcomes of 230 patients (148 males, mean age 43.5 years) affected with ulcerative colitis (UC, n = 194) or Crohn's disease (CD, n = 36) who underwent one or more cycles of GMA were analyzed. Each cycle consisted of five GMA treatments. The patients were followed up for a mean of 8.7 (min. 3 to max. 12) months. At 3 months, positive outcome was achieved in 77.7% of UC patients (72.0% remission, 5.7% clinical response) and 61.3% of CD patients (54.8% remission, 6.5% clinical response). The cumulative proportion of positive outcome at 12 months was 87.1% for UC patients (83.7% remission, 3.4% clinical response) and 77.4% for CD patients (74.2% remission, 3.2% clinical response). No single clinical or laboratory parameter among those analyzed (age, sex, disease characteristics, history of smoking, medication history, baseline values of clinical activity index (CAI)/Crohn's disease activity index (CDAI), hemoglobin, white blood cells count, and erythrocyte sedimentation rate) was independently associated with clinical outcome. The procedure was well tolerated with no significant adverse effects registered.