The objective of this study was to examine the efficiency of a novel method of early pregnancy diagnosis in cows. This method was based on crosssectional area changes of corpora lutea measured by transrectal ultrasonography. One hundred forty one Japanese Black cows were artificially inseminated and divided into four groups according to examination days. Corpora lutea of all cows in each group were measured twice at 7 days apart, on days 13 and 20 (n = 18), 14 and 21 (n = 40), 15 and 22 (n = 37) and 16 and 23 (n = 46) post-insemination. Confirmation of pregnancy was performed by ultrasonography on day 30 postinsemination. All non-pregnant cows showed clear regression of their corpora lutea at rates of 35.8, 43.7, 51.3, and 56.2% on days 20, 21, 22, and 23 postinsemination, respectively. Some ovarian disorders were discovered in 9% of non-pregnant cows. In conclusion, the present method was highly sensitive (100%) in detecting early non-pregnancies in the field, and also helpful in detecting ovarian disorders at an early stage, which gives reason to use it for better economic management and benefits.