The treatment of ulcerative colitis has improved since the appearance of Tumor Necrosis Factor (TNF)-alpha inhibitors. However, the use of TNF-alpha inhibitors increases the risk of opportunistic infections. We describe two cases of Pneumocystis jiroveci pneumonia during infliximab therapy for active ulcerative colitis. They were successfully treated with sulfametoxazole/trimetroprim. High awareness of P. jiroveci pneumonia in patients who develop pulmonary symptoms with hypoxia during TNF-alpha modulator therapy is recommended.