Initial results are presented from a Lincoln Laboratory study of ACAS performance on the Global Hawk UAV. The study has been applying the process outlined in the ICAO ACAS Manual which involves developing UAV airspace encounter models and running fast-time Monte Carlo simulations of encounters. ACAS performance was examined in conventional aircraft vs. conventional aircraft, conventional aircraft vs. non-ACAS Global Hawk, and conventional aircraft vs. ACAS-equipped Global Hawk cases. The existing ICAO and ACASA encounter models were modified to reflect Global Hawk flight characteristics. ACAS performance on Global Hawk was also assessed parametrically across reaction latencies from 0 - 20 s. Global Hawk flight characteristics were shown to have a small but measurable negative impact on collision risk. Assuming no system failures or visual acquisition effects occur, performance with ACAS on Global Hawk is significantly better than without ACAS if response latencies (from the moment an RA is issued to the moment maneuvering begins) are less than 10 s. Performance drops off rapidly at latencies greater than 10 s. The needs for improved airspace models and a more in-depth study of the interaction between visual acquisition and ACAS are noted.