The literature pertaining to the prevalence of avian hematozoa in North America, north of Mexico, together with unpublished records from the files of the International Reference Centre for Avian Malaria Parasites, have been collated and analyzed with respect to the distribution of parasite genera by region, host family, and vertical stratification of nesting sites. The blood parasites of 57 026 birds of 388 species (representing 55 families) breeding in North America are the basis of this analysis. A total of 21 048 (36.9%) birds harbored one or more species of Haemoproteus (19.5%), Leucocytozoon (17.7%), Trypanosoma (3.9%), Plasmodium (3.8%), microfilariae (3.1%), and Haemogregarina/Lankesterella (0.6%). The continent was arbitrarily divided into seven regions along a topographic basis. Overall hematozoan prevalence varied markedly from region to region, as did the relative frequencies of the different genera. Overall hematozoan prevalence also varied markedly from avian family to family, as did the relative frequencies of the different genera. Nine families had prevalences of 50% or more, while 16 families had prevalences of 5% or less; sea- and shore-birds were nearly hematozoan-free. Prevalence of blood parasites is correlated with the vertical stratification of nesting sites when considered on a localized geographic basis but no correlation exists when the results are pooled from the continent.