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Abstract Background Recent studies have reported that reduced dose trimethoprim-sulfamethoxazole (TMP-SMX) may be effective in the treatment of Pneumocystis jirovecii pneumonia (PJP) but data is lacking for patients with hematologic malignancies. Methods This retrospective study included all adult hematologic patients with PJP between 2013 and 2017 at six Swedish University Hospitals. Treatment with 7.5-15 mg TMP/kg/day (reduced dose) was compared with >15-20 mg TMP/kg/day (standard dose), after correction for renal function. The primary outcome was the change in respiratory function (ΔPaO2/FiO2) between baseline and day 8. Secondary outcomes were clinical failure and/or death at day 8 and death at day 30. Results Out of a total of 113 included patients, 80 patients received reduced dose, and 33 patients received standard dose. The overall 30-day mortality in the whole cohort was 14%. There were no clinically relevant differences in ΔPaO2/FiO2 at day 8 between the treatment groups, neither before nor after controlling for potential confounders in an adjusted regression model (-13,6 mmHg [95% CI -56,7-29,5] and -9,4 mmHg, [95% CI -50.5-31.7], respectively). Clinical failure and/or death at day 8 and 30-day mortality did not differ significantly between the groups, 18% vs. 21% and 14% vs. 15%, respectively. Among patients with mild to moderate pneumonia, defined as PaO2/FiO2>200 mmHg, all 44 patients receiving reduced dose were alive at day 30. Conclusion In this cohort of 113 patients with hematologic malignancies, reduced dose TMP-SMX was effective and safe for treating mild to moderate PJP.