Both influenza and meningococcal disease (MD) show seasonal variation with peak incidence rates during the winter. We examined whether fluctuations in occurrence of influenza were associated with changes in the incidence rate of MD, either simultaneously or with a delay of one or 2 weeks, and whether age had an impact on these associations. This ecological study was based on weekly surveillance data on influenza and a complete registration of MD cases (n = 413) in North Jutland County, Denmark, during 1980-1999. A total of 379 MD cases occurred during weeks with influenza registration. The analysis was done using a Poisson regression model taking into account the seasonal variation and trend over time in incidence rate of MD, and stratified by age: < 1 year (n = 38), 1-14 years (n = 189), and > or = 14 years (n = 152). An increase of 100 registered cases of influenza per 100,000 inhabitants was associated with a 7% (95% CI: -1 to 15%) increase in the number of MD cases during the same week. The association was most marked for < 1 year-olds, corresponding to a 29% (95% CI: 6-58%) increase in the number of MD cases per 100 cases of influenza per 100,000 inhabitants. Our findings support the theory that the influenza detection rate is associated with the number of MD cases in the population during the same week.