Objective: To compare health care utilization changes between Medicare beneficiaries with two prescription drug benefit structures who did and did not reach their respective Part D drug benefit spend threshold in 2006. Methods: A retrospective analysis of a cohort of 28,392 Medicare Advantage beneficiaries continuously enrolled into two distinct drug benefit structures for the year prior to and after implementation of the Medicare Part D benefit. The first benefit group (Silver) had the Part D standard drug benefit threshold ($2250) while the second (Gold) had an enhanced benefit with a higher threshold ($3500). Poisson, zero-inflated Poisson, and negative binomial modeling were used to compare post-implementation utilization rates with adjustment for pre-period utilization, demographics and morbidity burden. Results: A total of 1237 (6%) Silver and 526 (8%) Gold beneficiaries reached their threshold. Among both groups, beneficiaries who reached their threshold had greater morbidity burden and higher rates of pre-period inpatient admissions and medical office and ED visits (all p<0.001). Among beneficiaries who reached their threshold, there was no change in inpatient and ED (both p>0.05) but an increase in medical office visit (p<0.001) utilization rates in comparable 6-month periods before and after reaching their threshold. Sub-analyses indicated that beneficiaries with the highest morbidity burden experienced higher utilization rates (all p<0.05), but there were no differences between groups (all p>0.05). Beneficiaries in both groups who did reach their threshold had higher post-period utilization rates (all p<0.001) regardless of age and morbidity burden and were more likely to die (p<0.001) compared to beneficiaries who did not reach their threshold. Conclusion: Although many Medicare beneficiaries navigate their drug spend threshold without experiencing increased healthcare utilization, those with high morbidity burdens are at risk of increased health care utilization and the potential for adverse outcomes. It is imperative that strategies be developed that help safeguard vulnerable Medicare beneficiaries.