The dwarf syndrome in wheat crops in Bulgaria was first described almost a century ago. Nowadays several viruses are known as pathogens able to trigger the above mentioned syndrome. Among them the most important ones from the economic point of view are the barley yellow dwarf virus (BYDV) and the wheat dwarf virus (WDV). Under natural conditions the first one is transmissible by aphids. For the second one some leafhopper species are detected as natural vectors. The infected plants fall behind in their development. Ordinarily they either do not form spikelets or if they do they are underdeveloped with poor quality grains. Based on investigations carried out during the period 1997-2001 a procedure for evaluation of the viral infection was created. It contains two main steps: categorizing of invasion and spread of the virus in cereal crops as well as the type of reaction of the infected plants.