Wavelet power spectral analysis of the inpatient and outpatient time series identified statistically significant oscillatory modes across different years and at all periodic scales examined* indication of the endemic nature of dengue in PR. Crossspectral analysis of the two time series revealed three distinct areas of significant coherence as seen in Fig. 1 . At the 1-year period, the two time series have strong association during 2004Ápresent, with waves of outpatient cases lagging behind those of inpatient cases by an average lag time of 7 days (not shown). At the 1.5-year period, the two time series cohere during 1999Á2003, indicating that dengue outbreaks did not occur yearly in the VHA population prior to 2004. At the 3.5Á4-year period, they cohere across all years in the data, an indication of a multiyear interepidemic period.