Trimethoprim-sulfamethoxazole (TMP/SMX) is used as prophylaxis against Pneumocystis jiroveci during chemotherapy. Many groups recommend withholding TMP/SMX during high-dose methotrexate (HDMTX) for concerns that it will delay methotrexate clearance. We compared methotrexate exposure following HDMTX (NCT00549848) in 424 patients including 783 courses that were given concurrently and 602 courses that were not given concurrently with TMP/SMX. Among 176 patients (555 courses) on the low-risk arm (HDMTX=2.5 g/m/24 h), there was no difference in clearance (110.7 [1.8%] vs. 108.2 [0.9%] mL/min/m, P=0.3) nor in 42 hour methotrexate concentration (0.37 [5.1%] vs. 0.40 (5.0%) μM, P=0.23). Among 248 patients (830 courses) on the standard/high-risk arm (HDMTX ~5 g/m/24 h), there was slightly higher clearance (95.5 [1.4%] vs. 91.2 [0.8%] mL/min/m, P=0.005) in those receiving TMP/SMX, with no difference in the 42 hour methotrexate concentration (0.59 [4.1%] vs. 0.66 [4.2%] μM, P=0.06). There was no difference in neutrophil counts based on TMP/SMX during HDMTX (P=0.83). TMP/SMX also did not have a significant impact on myelosuppression of low-dose methotrexate (40 mg/m) given during continuation therapy among 230 patients enrolled on a prior study (NCT00137111). Thus, we found no evidence for an interaction between methotrexate and TMP/SMX given prophylactically.