Ketosis is a metabolic syndrome that can increase farm costs and result in milk production losses during a lactation. This syndrome mostly affects high production cows and causes are usually associated with negative energy balance during early to mid-lactation. The objective of this study was to evaluate the concentration of ketone bodies in the blood of 732 early-lactation [25.2 ± 15.8 d in milk (DIM)] high producing Holstein cows (32.98 kg/d of milk yield) from 10 intensively managed farms in Brazil. A portable electronic diagnosis equipment (Ketovet, TaiDoc technology, Taiwan, China) was used for measurement of blood ketone bodies and results are expressed in mM. Blood was sampled from the tail’s artery or vein, and the volume of 1 drop was added to a reagent strip set for β-hydroxybutyrate (BHBA) analysis and 5 s was the average time needed for the equipment to yield results. Animals with values of blood BHBA of less than 1.1 mM were considered normal and free of metabolic syndrome. Animals with values between 1.2 and 3.5 mM were diagnosed with subclinical ketosis, and animals with values above 3.5 were diagnosed with clinical ketosis. Animals were blocked by days in milk (0–15, 16–30, 31–45, >46) and farm was used as a random effect. Data were analyzed using a t student test of least square difference and significance was declared as P < 0.05. From the 732 animals evaluated, 448 (47.85%) were considered free from the syndrome, 279 (38.11%) had subclinical ketosis and 5 (0.68%) had clinical levels of BHBA. Similar BHBA levels were found for cows between 0 and 15 DIM (1.07 mM), between 16 and 30 DIM (1.15 mM) and between 31 and 45 DIM (1.06 mM), but were significantly higher for cows with more than 46 DIM (1.34 mM, SEM = 1.34, P < 0.01). Results of this experiment suggest that cows with more than 46 DIM had subclinical ketosis and values of BHBA that were higher than for all cows with less than 45 DIM.