The fertility of high yielding dairy cows has decreased during the last 25 years. For example, the calving-interval(CI) has extended with 30 days (from 390 days in 1992 to 420 days in 2008). Consequently, practitioners dealing withherd health management, are often faced with farmers’ questions about the financial impact of an extended CI.Numerous studies have attempted to calculate the economic consequences of an extended CI. Some researchersbased their investigations on real data and analyzed them retrospectively, while others simulated situations by complexmodels. Undoubtedly, the lactation curve is the most important contributor to these calculations, besides a whole rangeof other factors, which may influence the economic results in a positive or negative way. Consequently, conclusionsare not always that unambiguous. In general the conclusion is that an extended CI will result in a loss of money.Extending the CI from 395 days to 432 days for instance results in a loss of € 2.08 per cow per day; in total € 77 percow (37 days).Because the lactation persistency is rather plastic, some managerial measures, such as a higher milking frequencyand the use of bovine somatotropin, can be taken to increase the persistency resulting in a less negative or even apositive economic balance.