Two naturally-ventilated freestall dairy barns, constructed in the mid-western Un ited States in the mid 1990's, were recently (spring, 2007) retrofitted into mechanical "cross-flow" ventilation systems to improve cow comfort and performance. The two barns are part of the National Air Emission Monitoring Study (NAEMS), which is measuring gas and particulate emissions from livestock and poultry buildings and associated manure storages. The two, approximately 27-meter-wide barns were converted from natural ventilation to cross-flow ventilation systems by installing a continuous row of belt-driven, 127-cm diameter axial exhaust fans (59 and 66 fans respectively for barns 1 and 2) along one side of each barn. Air enters each barn along the opposite sidewalk. A solid plastic curtain baffle runs the length of both barns from the closed ridge peaks to the top of a row of headlocks along the feed manger to force ventilation air down to cow level. In addition, two rows of water fogging nozzles (I m spacing) were installed in both barns, one at the sidewall inlets and the other at the bottom of the interior baffle. The fogging nozzles provide evaporative cooling to the incoming and cross ventilating air. The new ventilation systems were able to maintain indoor air below ambient outdoor temperatures in the two dairy barns and provided convective cooling of the cows during warm summer days. Indoor air quality as measured by carbon dioxide and ammonia concentration were well within generally acceptable levels. Milk yield and feed efficiency improved in these two barns during the 4 summer (June - September) months of 2007 compared to the previous three summers (2004-2006) even though 2007 had greater number of days above 30 °C. Additional research is needed to show more statistical evidence of the biological improvements in herd performance from cross-flow ventilation /evaporative cooling systems.