Haematology can provide useful information to assess the health of free-ranging animals. In seabirds, many procellariiform (albatrosses and petrels) species undertake long-distance migrations during which individuals may get in contact with birds and pathogens of different biogeographic origin. Although checking whether procellariiformes might play a role in large-scale spread of diseases is desirable, haematological data have been published in very few species. For the first time, we determined blood cell counts in Cory's shearwater, Calonectris diomedea, a petrel which breeds in the western Palearctic but overwinters off Brazil and South Africa. Forty-five adult Cory's shearwaters were captured in their burrows in the Azores archipelago during the pre-laying period in March 2008, measured, weighed, and a blood sample was taken from each of them. Haematocrit, polychromasia, leukocyte and thrombocyte counts did not differ significantly between sexes, but total leukocyte number was significantly (positively) correlated with body condition. [ABSTRACT FROM AUTHOR]