We report the clinical and laboratory findings concerning three unrelated Brazilian patients investigated for polycythemia, whose definitive diagnosis could only be established after the presence of Hb Coimbra (β99 Asp → Glu) was demonstrated. This illustrates the importance of properly investigating hereditary hemoglobinopathies in cases of erythrocytosis because in some populations variants with high oxygen affinity may be more frequent than expected but go undetected when conventional electrophoresis is used as the sole detection procedure. Copyright by the Brazilian Society of Genetics.292200202Antonarakis, S.E., Boehm, C.D., Giardinia, P.J., Kazazian Jr., H.H., Nonrandom association of polymorphic restriction sites in the beta-globin gene cluster (1982) Proc Natl Acad Sci USA, 79, pp. 137-141Bunn, H.F., Forget, B.G., (1986) Hemoglobin: Molecular, Genetics and Clinical Aspects, p. 690. , WB Saunders Company, PhiladelphiaChanutin, A., Curnish, R.R., Effect of organic and inorganic phosphates on the oxygen equilibrium of human erythrocytes (1967) Arch Biochem Biophys, 121, pp. 96-102Park, S.S., Kim, J.Y., Cho, H.I., RFLP haplotypes of betaglobin gene complex of beta-thalassemic chromosomes in Koreans (2002) J Korean Med Sci, 17, pp. 475-478Miranda, S.R.P., Fonseca, S.F., Figueiredo, M.S., Yamamoto, M., Grotto, H.Z.W., Saad, S.T.O., Costa, F.F., Hb Köln [a 2β 2 98 (FG) Val → Met] identified by DNA analysis in a Brazilian family (1997) Braz J Genet, 20, pp. 745-748Nagai, K., La Mar, G.N., Jue, T., Bunn, H.F., Proton magnetic resonance investigation of the influence of quaternary structure on the iron-histidine bonding in deoxy hemoglobins (1982) Biochemistry, 21, pp. 842-847Orkin, S.H., Kazazian, H.H.J., Antonarakis, S.E., Goff, S.C., Boehm, C.D., Sexton, J.P., Waber, P.G., Giardina, P.J., Linkage of beta-thalassemia mutations and beta-globin gene polymorphisms with DNA polymorphisms in human beta-globin gene cluster (1982) Nature, 296, pp. 627-631Reed, C.S., Hampson, R., Gordon, S., Jones, R.T., Novy, M.J., Brimhall, B., Edwards, M.J., Koler, R.D., Erythrocytosis secondary to high oxygen affinity of a mutant hemoglobin, hemoglobin Kempsy (1968) Blood, 31, pp. 623-632Rossi-Fanelli, A., Antonini, E., Studies on the oxygen and carbon monoxide equilibria of human myoglobin (1958) Arch Biochem Biophys, 77, pp. 478-492Sutton, M., Bouhassira, E.E., Nagel, R.L., Polymerase chain reaction amplification applied to the determination of betalike globin gene cluster haplotypes (1989) Am J Hematol, 32, pp. 66-69Tamagnini, G.P., Ribeiro, M.L., Valente, V., Ramachandran, M., Wilson, J.B., Baysal, E., Gu, L.H., Huisman, T.H., Hb Coimbra or alpha 2 beta (2)99(GI)Asp → Glu, a newly discovered high oxygen affinity variant (1991) Hemoglobin, 15, pp. 487-496