OBJECTIVE: Establishing traditional intravenous (IV) access in adult trauma and medical patients can be difficult. We evaluated provider performance for obtaining intraosseous access with two FDA-approved intraosseous devices (F.A.S.T.1 and EZ-IO) in two sequential field trials. METHODS: One hundred twenty-four providers consented to participate in the first field trial evaluating the use of the F.A.S.T.1 system. Three hundred eighty-nine providers consented to participate in the second field trial, evaluating the use of the EZ-IO. Following each insertion attempt, a telephone data collection process with a member of the research team was completed. Insertion success rate and measures of provider comfort and satisfaction with each device were collected and analyzed. RESULTS: One hundred seventy-eight insertions (89 F.A.S.T.1; 89 EZ-IO) were completed between February 2000 and December 2005. Sixty-four of the 89 insertions of the F.A.S.T.1 were successful, and 78 of the 89 insertions of the EZ-IO were successful (72% vs. 87%; chi2 = 6.8; p = 0.009). Providers using the F.A.S.T.1 attempted more IV insertions prior to using the IO device than the providers using the EZ-IO (2.6 vs. 2.0, p = 0.005). There were no differences in provider comfort or provider assessed device performance between the two devices (p = 0.52; p = 0.13, respectively). CONCLUSION: In our comparison of two field trials of prehospital provider use of the F.A.S.T.1 and EZ-IO systems, more successful insertions with the EZ-IO were achieved than with the F.A.S.T.1 device. Limitations of our comparison include nonrandomization, the sequential field trial design, the potential for a learning effect, and self-reporting of data points by providers. A prospective, randomized evaluation of these devices is warranted to draw definitive conclusions about provider insertion success rate with these devices.