The body condition score (BCS) is a practice and common approach for evaluating the nutritional status of dairy cows which, however, has been questioned because of its subjectivity. Alternatively, backfat thickness (BFT) is a reliable parameter to assess the change of the energy status of dairy cows and can be objectively measured by ultrasonography. A high correlation between BCS index and BFT has been obtained in Holstein dairy herds where cows were fed with total mixed ration. The objective of the present study was to determine the relationship between these variables as well as how much they vary throughout the lactation of cows in a grazing-based system. Paired measurements of BCS and BFT were performed monthly from October 2017 to September 2018 in 128 cows of a commercial herd in Southern Brazil. The diet of cows was based on grazing tropical grasses from October to April and temperate grasses from May to September, supplemented at variable rates with concentrate, maize silage and/or grass hay. BCS was estimated using a 5-point scale and BFT (mm) was obtained using a 5-MHz linear transducer positioned vertical to a line between the tuber coxae and tuber ischia at the sacral site. Both variables were measured 4 to 7 repeated times in each cow throughout the experimental period. Pearson correlation and linear regression analysis were performed on data set. The minimal and maximal values of the variables were: DIM: 7 to 303; milk production (L/d): 6.3 to 36.0; BCS: 1.25 to 4.00 and BFT (mm): 2.8 to 30.4. There was a significant (P < 0.05) low correlation (r) between DIM and either BCS (r = 0.23) or BFT (r = 0.27) as well as between BCS and BFT (r = 0.33). The linear regression between BFT and BCS, which model included the cows as a random class variable, was: BFT (mm) = 2.3 + 5.2BCS (P < 0.05, RMSE = 2.79). In conclusion, in a grazing-based system where herbage allowance and quality are broadly variable throughout the year, the change in either BCS or BFT of cows was only weakly related to their lactation stage. Moreover, the change in BFT of cows was not accurately detected as a change in their BCS.