This paper reports a case study comparing the relative efficiency of using a Diabetes Registry or a Clinical Data Warehouse to recruit participants for a diabetes clinical trial, TECOS. The Clinical Data Warehouse generated higher positive predictive accuracy (31% vs. 6.6%) and higher participant recruitment than the Registry (30 vs. 14 participants) in a shorter time period (59 vs. 74 working days). We identify important factors that increase clinical trial recruitment efficiency and lower cost.