OBJECTIVES: Chronic lymphocytic leukemia (CLL) accounts for almost 40% of all leukemias. Current treatments have high rates of adverse events requiring hospitalization.  With promising treatments on the horizon, the need for well-designed studies of treatment patterns and adverse events will increase.  Designing studies can be challenging given the long-term nature of CLL. This study uses an online analytic tool to explore the necessary observation period to accurately assess treatment and hospitalization rates. METHODS: Using the Treatment Pathways tool and data from an 8-year oncology subset of the 2004 – 2012 MarketScan®databases, we identified patients with two plus claims for CLL, one year prior enrollment, no prior treatment.  Four follow-up groups were assessed:  1, 2, 3, and 4 years of continuous enrollment (CE).  For each CE, we identified patients treated with bendamustine (B), or fludarabine,  rituximab, and/or cyclophosphamide (F/R/C). Treatment and hospitalization rates and the time between diagnosis, treatment, and hospitalization were calculated. RESULTS: A total of 4886 patients met all inclusion criteria; 3348 had 1 year, 2201 had 2 years, 1451 had 3 years, and 874 had 4 years CE.   Bendamustine use increased from 4% among those with 1 year CE to 5% for all other CE groups.  F/R/C use increased from 21% among those with 1 year to 27% among those with 4 years CE.  Hospitalization rates increased from 41% to 49% for bendamustine, and 38% to 44% for F/C/R from 1 year to 4 years CE.  Among those with 4 yrs CE, median time to first treatment was 4.3 years for bendamustine, 1.4 years for F/C/R; median time to first hospitalization was 96 and 365 days, respectively.   CONCLUSIONS: This study used an online tool to quickly assess the impact of various CE criteria. The data demonstrate how shorter CE underestimates treatment, related hospitalizations, and overall burden of illness in a chronic population.