For three successive years (1976-1979), the community surveillance system of the Influenza Research Center identified a wave of influenza virus infections which occurred during the latter half of one epidemic period and which heralded the epidemic virus for the following year. Recognition of the herald waves required consistent and continuous sampling of respiratory illnesses occurring among persons who constituted a substantial proportion of the population of over two million people in Harris County, TX. The herald wave infections consisted of only 0.4-2.0% of the 2500 to 3000 respiratory illnesses examined for virus. During the first two epidemic periods, the herald wave was easily detected as a wave of influenza B virus infections occurring during an influenza A virus epidemic and then a wave of influenza A virus infections during an influenza B virus epidemic. The third herald wave was obscured because it consisted of a small number of infections caused by viruses antigenically like A/Brazil/11/78(H1N1) which occurred at the end of the outbreak caused by influenza A/USSR/90/77 (H1N1). The herald wave infections were detected among persons who lived in widely scattered areas of the county and who were representative of all age and socioeconomic groups. the herald wave phenomenon has the obvious benefit of providing information about the virus most likely to produce an epidemic during the next respiratory disease season. Under optimal conditions, this could allow six to nine months to prepare and administer control measures.