Crohn's disease and ulcerative colitis (UC) are common, chronic inflammatory bowel diseases (IBDs) characterized by episodes of life-altering symptoms such as diarrhea, bleeding, fecal urgency and incontinence, abdominal pain and cramps, and fever lasting weeks to months at a time. Existing treatments are 5-aminosalicyclates or immunosuppressants, but long-term control of IBD is a major problem for a large number of patients. Phosphodiesterase 4 (PDE4) is a key enzyme in cell homeostasis and inflammation and its inhibition has been useful in diseases such as asthma and chronic obstructive pulmonary disease, rheumatoid arthritis and multiple sclerosis. This review focuses on the role of oxidative stress in IBD and the PDE4 inhibitor OPC-6535 (tetomilast), an investigational agent for the treatment of UC. The authors detail the clinical development of the compound and report and provide insight into some of the unpublished data from the recently completed multicenter Phase III trials in UC.