Summary In 480 patients with hypsarrhythmia (H) and also in a matched control group (180), evidence for some recent infection was determined as a possible etiology for the H. Evidence for infection was found in 38% of the H patients, significantly (p<0.0005) different from the 23% in the control group. Although the H group more often showed specific infections with a known organism, a greater difference was seen in nonspecific infections with temperatures of at least 103° F. These data provide further evidence that infectious disease has likely played a more significant role in H than has been previously appreciated.