An increasing incidence of Pneumocystis jiroveci pneumonia (PCP) in patients with B-cell non-Hodgkin lymphoma (B-NHL) receiving rituximab treatment has been reported. We reviewed patients with B-NHL who underwent chemotherapy from 2004 to 2008 at our institution to identify risk factors for PCP development during and after chemotherapy. Among 297 patients with B-NHL, six developed PCP. Of 121 patients (41%) who received PCP prophylaxis with sulfamethoxazole–trimethoprim during chemotherapy, none developed PCP (0%), while among 176 patients (59%) who had no prophylaxis, six (3.4%) developed PCP at a median of 2 months (range: 1–3 months) after starting chemotherapy. Patients with CD4+ lymphocyte counts ≤200/mm3 before chemotherapy had a higher risk of developing PCP (p=0.045), while a history of rituximab treatment was not related to PCP. CD4+ lymphocyte counts ≤200/mm3 during and after chemotherapy were observed in 18.9% of patients.