BACKGROUND : Practice patterns of same-day versus next-day pegfilgrastim vary in numerous practice settings across the country. Current utilization with same-day pegfilgrastim reduced overall visits and reduced treatment time for chemotherapy administration. OBJECTIVE : To assess the efficacy and safety of same-day versus next-day pegfilgrastim in patients with colorectal cancer. METHODS : Patient data was extracted through electronic health records (EHR) search of ICD-9 codes that matched patients with CRC and treated with FOLFOX or FOLFIRI from November 2013 to January 2016. The incidence rates of primary and secondary endpoints were estimated for patients who received either FOLFOX or FOLFIRI and same-day pegfilgrastim with 2-sided 95% confidence intervals. Fisher's exact test for 2 × 2 contingency tables was used to compare the incidence of primary and secondary endpoints between the two study groups performed at the α = 0.05 significance level. A study by Hecht et al. served as a historical control for next-day pegfilgrastim. RESULTS : A total of 109 out of an initial 330 patients with appropriate ICD-9 criteria were eligible for study inclusion. The primary endpoint of incidence of FN recorded over 4 chemotherapy cycles with either FOLFOX6 or FOLFIRI occurred in 3.7% of patients (95% CI, 1.1-9.4%). Secondary endpoints also occurred with a relatively low incidence: 13 patients developed grades 3/4 neutropenia (11.9%; 95% CI, 7.0-19.5%); 11 patients required dose reductions because of neutropenia or FN (10.1%; 95% CI, 5.6-17.3%); and 5 patients were hospitalized due to neutropenia or FN (4.6%; 1.7-10.6%). There were 4 reported events of FN (3.2%; 95% CI, 1.0-8.3%) for those who received next-day pegfilgrastim compared to 11 events in the placebo group (9.4%; 95% CI, 5.1-16.4%). The incidence of dose delays or dose reductions due to neutropenia or FN were 5 (4.1%, 95% CI, 1.5-9.4%) in the next-day pegfilgrastim group versus 26 (22.1%, 95% CI, 15.5-30.4%) in the placebo group. CONCLUSIONS : The study was retrospective in design and utilized a historical control for the comparator. CONCLUSIONS : Our study results suggest that same-day pegfilgrastim administration may be a safe and effective alternative to 24-h post-chemotherapy administration in patients with esophageal, gastric, appendiceal, or colorectal cancer undergoing treatment with FOLFOX or FOLFIRI.