BACKGROUND: Prior epidemiological and intervention studies have not been able to separate independent effects of dose, timing, and duration of aspirin use in colorectal cancer (CRC) chemoprevention. We examined aspirin-based CRC chemoprevention according to timing in the Nurses' Health Study and Health Professionals Follow-Up Study. METHODS: The exposures include cumulative average dose and total duration of aspirin use in more than 10 years before follow-up started (remote period) and in the immediate 10 years before follow-up started (recent period). Cox models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for exposures and CRC risk. RESULTS: Aspirin use of longer than 10 years before follow-up started (HR = 0.88, 95% CI = 0.83 to 0.94) per 5-year increment and the immediate 10 years before follow-up started (HR = 0.90, 95% CI = 0.84 to 0.96) were similarly important in CRC chemoprevention, though a 5-year lag was required for a clear benefit in the recent period. In the remote period, the association was not dose dependent; compared with less than 0.5 standard-dose (325 mg) tablets per week; hazard ratios were 0.78 (95% CI = 0.63 to 0.98), 0.81 (95% CI = 0.72 to 0.91), and 0.74 (95% CI = 0.64 to 0.86) for doses of 0.5 to less than 1.5, 1.5 to less than 5, and 5 and more tablets per week, respectively. However, there was dose dependency in the recent period (with respective HR = 0.91, 95% CI = 0.79 to 1.06; HR = 0.87, 95% CI = 0.77 to 0.98; and HR = 0.76, 95% CI = 0.64 to 0.91). CONCLUSIONS: A suggestive benefit necessitates at least 6-10 years and most clearly after approximately 10 years since initiation of aspirin. Remote use and use within the previous 10 years both contribute independently to decrease risk, though a lower dose may be required for a benefit with longer term use.