Problems with poorly documented immunization records may be especially important in rural areas. To evaluate the potential impact of a regional registry in a rural region, this study quantified the change in documented immunization rates for nine primary care sites in rural Colorado resulting from the addition of public health department immunization clinic records. Manual chart reviews of immunization data were conducted at both private primary care and public health department sites in two geographic areas in rural Colorado. Data from private primary care sites were matched to data from the public health department sites. Immunization up-to-date (UTD) rates at each primary care site were then recalculated for 12- and 24-month-olds after including data from public health department sites. Of 1,533 children, 469 (31 percent) were given immunizations at both a private primary care and a public health department site. The UTD rate (3:2:3:2) of 12-month-olds using only data from primary care sites ranged from 32 to 79 percent. Including the public health department data increased the rates by 0 to 26 percent (mean = 11 percent) for 12-month-old children. The UTD rate of 24-month-olds (4:3:1:3 and any Hib on/after 12 months) ranged from 6 to 54 percent at the primary care sites. These rates increased by 6 to 21 percent (mean = 12 percent) when public health department data were added. This "virtual" registry combining primary care and public health department data increased calculated immunization rates at primary care sites substantially, with a range of 0 to 26 percent.