Estimates of the proportions of the population who are at high risk of influenza complications because of prior health status or who are likely to have decreased vaccination response because of immunocompromising conditions would enhance public health planning and model-based projections. We estimate these proportions and how they vary by population subgroups using national data systems for 2006-2008. The proportion of individuals at increased risk of influenza complications because of health conditions varied 10-fold by age (4.2% of children 64 years). Age-specific prevalence differed substantially by gender, by racial/ethnic groups (with African Americans highest in all age groups) and by income. Individuals living in families with less than 200% of federal poverty level (FPL) were significantly more likely to have at least one of these health conditions, compared to individuals with 400% FPL or more (3-fold greater among 64 years). Among children, there were significantly elevated proportions in all regions compared to the West. The estimated prevalence of immunocompromising conditions ranged from 0.02% in young children to 6.14% older adults. However, national data on race/ethnicity and income are not available for most immunocompromising conditions, nor is it possible to fully identify the degree of overlap between persons with high-risk health conditions and with immunocompromising conditions. Modifications to current national data collection systems would enhance the value of these data for public health programs and influenza modeling.