Systematic virologic surveillance in Houston over the past seven years has revealed that influenza virus infections have been epidemic during each respiratory disease season. The peak of the occurrence of acute respiratory illnesses that caused patients to seek medical care always coincided with the peak of influenza virus activity. The peak number of deaths attributed to pneumonia or influenza followed the peak of influenza virus activity by about two weeks. This relationship was constant even though the time of the epidemic peak ranged from as early as the third week of December to as late as the second week of March. No increase in deaths occurred in the absence of influenza virus activity. Therefore, the methods used to predict base-line mortality that show a seasonal rise in the absence of influenza activity may be inaccurate and, if so, lead to underestimation of mortality associated with influenza virus infections.