Background: A wide variety of myelosuppressive chemotherapy regimens are used for the treatment of cancer in clinical practice. Neutropenic complications, such as febrile neutropenia, are among the most common side effects of chemotherapy, and they often necessitate delays or reductions in doses of myelosuppressive agents. Reduced relative dose intensity (RDI) may lead to poorer disease-free and overall survival. Methods: Using the McKesson Specialty Health/US Oncology iKnowMed electronic health record database, we retrospectively identified the first course of adjuvant or neoadjuvant chemotherapy received by patients without metastases who initiated treatment between January 1, 2007, and March 31, 2011. For each regimen, we estimated the incidences of dose delays (≥7 days in any cycle of the course), dose reductions (≥ 15% in any cycle of the course), and reduced RDI (<85% over the course) relative to the corresponding standard tumor regimens described in the NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines). Results: This study included 16,233 patients with 6 different tumor types who received 1 of 20 chemotherapy regimens. Chemotherapy dose delays, dose reductions, and reduced RDI were common among patients treated in community oncology practices in the United States, but RDI was highly variable across patients, regimens, and tumor types (0.486–0.935 for standard tumor regimen cohorts). Reduced RDI was more common in older patients, obese patients, and patients whose daily activities were restricted. Conclusions: In this large evaluation of RDI in US clinical practice, physicians frequently administered myelosuppressive agents at dose intensities lower than those of standard regimens.