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The first 2 weeks after calving have been described as the main risk period for hyperketonemia (HYK). However, it was recently shown that dairy cows continued to be at risk for HYK until at least 42 DIM.The objectives of our study were to describe the occurence of hyperketonemia (HYK) within the first six weeks of lactation and to evaluate the effects of HYK on milk production, reproductive performance and early lactation culling risk. A total of 655 dairy cows from 6 commercial dairy farms in Germany were enrolled between 1 and 4 DIM. Cows were tested twice weekly using an electronic handheld meter for β-hydroxybutyrate (BHBA) for an examination period of 42 days resulting in 12 test results per cow. Hyperketonemia was defined as a BHBA concentration ≥ 1.2 mmol/l. The onset of HYK was described as early onset (first HYK event within the first 2 weeks) and late onset (first HYK event in week 3 to 6 postpartum). The effects of onset of HYK within the first six weeks of lactation on milk production (1st test day milk yield and 100 DIM milk yield), reproductive performance (time to first service, first service conception risk and pregnancy risk within 200 DIM) and on early lactation culling risk were analyzed using a generalized linear mixed model. Cumulative incidence of HYK was 48% and 72% for primiparous and multiparous cows, respectively. Peak incidence was in week 1.0 for primiparous cows and in week 2.5 for multiparous cows. Mean prevalence was 17.5%. In total, 72% of HYK positive cows had a negative test result 3 to 4 days later after the initial diagnosis of HYK. Cows with early onset of HYK had a higher 1st test day milk yield (+ 3.0 kg/d, P < 0.001) and 100 DIM milk production (+ 301.6 kg; P < 0.001) compared to non-ketotic cows. There was no effect of late onset of ketosis on 1st test day milk yield and 100 DIM milk production. There were no effects of HYK on the time to first service, first service conception risk and pregnancy risk within 200 DIM, irrespective of onset of ketosis and no effect on culling risk. In total, 72% of HYK positive cows had a negative test result after the initial diagnosis of HYK which should be considered at future treatment studies. Cows with early onset of HYK had a higher milk production compared to non-ketotic cows. HYK was not associated with negative effects on reproduction and early lactation culling risk. HYK in early lactation seems to be part of a physiological adaptational response to negative energy balance in transition dairy cows.