Since 2001, many state and local health departments have implemented automated systems to monitor healthcare use and to promptly identify and track epidemics and other public health threats. In 2007-08, we conducted case studies of selected events with actual or potential public health impacts to determine whether and how health departments and hospitals used these new systems. We interviewed public health and hospital representatives and applied qualitative analysis methods to identify response themes. So-called "syndromic" surveillance methods were most useful in situations with widespread health effects, such as respiratory illness associated with seasonal influenza or exposures to smoke from wildfires. In other instances, such as a tornado or hazardous material exposures, these systems were useful for detecting or monitoring health impacts that affected relatively few people, or they were used to affirm the absence of outbreaks following natural disasters or the detection of a potential pathogen in air samples. Typically, these data supplemented information from traditional sources to provide a timelier or fuller mosaic of community health status, and use was shaped by long-standing contacts between health department and hospital staffs. State or local epidemiologists generally preferred syndromic systems they had developed over the CDC BioSense system, citing lesser familiarity with BioSense and less engagement in its development. Instances when BioSense data were most useful to state officials occurred when analyses and reports were provided by CDC staff. Understanding the uses of surveillance information during such events can inform further investments in surveillance capacity in public health emergency preparedness programs.