Abstract Objective:To evaluate the impact of clinical decision support (CDS) tools on rates of vitamin D testing. Screening for vitamin D deficiency has increased in recent years, spurred by studies suggesting vitamin D’s clinical benefits. Such screening, however, is often unsupported by evidence and can incur unnecessary costs. Materials and Methods:We evaluated how rates of vitamin D screening changed after we implemented 3 CDS tools in the electronic health record (EHR) of a large health plan: (1) a new vitamin D screening guideline, (2) an alert that requires clinician acknowledgement of current guidelines to continue ordering the test (a “hard stop”), and (3) a modification of laboratory ordering preference lists that eliminates shortcuts. We assessed rates of overall vitamin D screening and appropriate vitamin D screening 6 months pre- and post-intervention. Results:Vitamin D screening rates decreased from 74.0 tests to 24.2 tests per 1000 members (P < .0001). The proportion of appropriate vitamin D screening tests increased from 56.2% to 69.7% (P < .0001), and the proportion of inappropriate screening tests decreased from 43.8% pre-implementation to 30.3% post-implementation (P < .0001). Discussion:To our knowledge, this is the first demonstration of how CDS can reduce rates of inappropriate vitamin D screening. We used 3 straightforward, inexpensive, and replicable CDS approaches. We know of no previous research on the impact of removing options from a preference list. Conclusion:Similar approaches could be used to reduce unnecessary care and decrease costs without reducing quality of care.