The laboratory diagnosis of resistance to activated C protein (APC-resistance) involves examination of the phenotype and genotype of this thrombophilia. For examination of the phenotype coagulation and chromogenic tests are used. Their essence is examination in the presence and absence of exogenous APC. While the result of the original coagulation examination of APC-resistance which uses the APTT principle is influenced by a number of factors, the sensitivity and specificity of the modification of this examination (dilution of the examined plasma sample by FV deficient plasma before making the test) in relation to detection of FV Leiden is almost 100% and eliminates the majority of limitations of the original examination. The chromogenic assessment of APC-resistance has similar advantages, however, it cannot differentiate between the heterozygous and homozygous form of FV Leiden. During examination of the genotype of subjects with APC-resistance the mutation of FV Leiden is detected in as many as 90%. The group of subjects with the phenotype of APC-resistance comprises in particular subjects with acquired APC-resistance caused by conditions which lead to a disbalance between procoagulation and anticoagulation proteins of haemostasis which influence the reactions of the applied laboratory examinations. The acquired phenotype of APC-resistance can be also associated with an increased risk of thrombosis and the clinical manifestations of this thrombophilia resemble the classical, FV Leiden conditioned APC resistance. Rarely also congenital causes of the phenotype of APC-resistance are encountered caused by another mutation than the Leiden mutation of gene FV. The concurrent examination of the patient's plasma with the original and modified coagulation test makes it possible to assess the inborn cause of APC-resistance (positive finding also in modified examination). The presence of FV Leiden is then confirmed by examination of the genotype by the polymerase chain reaction.