Background: Because of prolonged screening requirements, patient and time-dependent selection have been proposed as potential biases in clinical trials. The screening process may exclude patients with a need for emergent treatment (and a short period from diagnosis to treatment initiation [DTI]). We explored the impact of DTI on overall survival (OS) in a population-based cohort of patients with diffuse large B-cell lymphoma (DLBCL). Patients and Methods: Using population-based administrative databases in Ontario, Canada, we identified adults aged ≥18 years with DLBCL treated with rituximab-based chemotherapy for curative intent between January 2005 and December 2015. Cox regression and multivariable analyses were presented to evaluate the impact of time from DTI on OS, controlling for relevant covariates. Results: We identified 9,441 patients with DLBCL in Ontario; median age was 66 years, 53.6% were male, median number of comorbidities (Johns Hopkins aggregated diagnosis groups) was 10 (interquartile range [IQR], 8–13), and median DTI was 37 days (IQR, 22–61). Between treatment initiation and study end, 43% of patients died (median OS, 1 year; IQR, 0.4–2.8 years). Shorter DTI was a significant predictor of mortality (P<.001). Compared with the shortest DTI period of 0–18 days, those who commenced therapy at 19–29 days (hazard ratio [HR], 0.75; 95% CI, 0.68–0.84), 30–41 days (HR, 0.70; 95% CI, 0.63–0.78), 42–57 days (HR, 0.52; 95% CI, 0.46–0.58), and 58–180 days (HR, 0.52; 95% CI, 0.47–0.58) had improved survival. Increasing age (HR, 1.03; 95% CI, 1.03–1.04), male sex (HR, 1.23; 95% CI, 1.14–1.32), and increasing number of comorbidities (HR, 1.12; 95% CI, 1.11–1.13) were associated with inferior survival. Conclusions: Among patients with DLBCL, shorter DTI was associated with inferior OS. Therefore, DTI may represent a surrogate marker for aggressive biology. Clinical trials with lengthy screening periods are likely creating a time-dependent patient selection bias.