Malignant catarrhal fever (MCF) is a mostly fatal lymphoproliferative disease of cattle. In 1995 a PCR based method was introduced for the detection of the ovine herpesvirus 2 (OvHV-2), which is regarded as the causative agent of the sheep-associated form of the disease. This PCR can be regarded as a gold standard for the in vivo diagnosis of sheep-associated MCF in cattle (Müller-Doblies et al., 1998). This semi-nested PCR was now used as a reference test for the reassessment of diagnostic criteria in the clinical and post mortem diagnosis that could previously not be quantitated. Based on 83 suspected cases with a complete clinical record the clinical signs were weighted and grouped according to their sensitivity and specificity into lead signs indicative of MCF and frequently accompanying signs supportive for the diagnosis of MCF and general clinical signs that were less reliable for the diagnosis. Differential diagnoses are discussed, which are of particular significance due to their status as OIE list A diseases e.g. foot-and-mouth disease or rinderpest. 38 PCR confirmed cattle with MCF served for the quantitative analysis of organ lesions. For the post mortem diagnosis an essential set of organ samples is defined to permit a reliable histological diagnosis, as the gross pathology often did not give any indication for the diagnosis. These criteria should help to improve the diagnostic efficiency and to select the appropriate laboratory diagnostic procedures for MCF-suspected cattle.