Paper presented the results of field experiment that was carried out within 2003-2004 in three places with 10 potato cultivars. Total yield and share of market yield in total yield were estimated. Reasons of appearing the defect formation in tubers were investigated. The national monitoring concerning total and market yield was carried out in 2005. It was estimated that the market yield amounted 65-70% total yield. Apart from the tubers diminution, most frequent defects included defects caused by soil pests and invasion by common scab. Significant differentiation of the cultivars was stated with respect to the amount of market yield obtained.