In the paper the influence of daily milk yield in peak lactation, milk yield during 305 days after calving and number of cows’ calving on frequency of lactations longer than 305-day standard were analyzed. It was also estimated the influence of extended lactation on milk yield and milk composition. The research were carried out in a herd including 220 Polish Holstein-Friesian cows of Black-And-White variety. The animals were housed in a loose barn and were fed a balanced TMR. The research included 384 lactations, which were longer at least 1 day than 305-day standard, and were selected out from 488 complete lactations. It was found that frequency of extended lactations was 78.7%, and moreover the most frequently (25.0%) the period of extension lasted up to 30 days. During extension period milk production was higher by 2.5–43.0% in compare to milk yield in standard lactation. Lactation extending was slightly connected with milk yield per 1 day of complete lactation. Values of this trait were highest in the case of lactation extended maximally by 60 days.

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