The aim of this study was to determine the association of weight loss during the transition period with the expression of estrus. Sixty-seven Holstein cows were enrolled onto the study 2 wk before parturition. Body weight (BW) was determined 1 and 2 wk before expected calving date and again 5 wk postpartum. Beta-hydroxybutyrate (BHBA) and glucose concentrations were analyzed 2 wk postpartum. Cows were equipped with collar-mounted activity monitors (Heatime, SCR Engineering, Israel). Estrous expression was measured by an automated activity monitor (AAM) and quantified using 2 parameters: 1) peak activity and 2) duration of the estrus episode. Peak activity was defined as the maximum activity index during an estrus episode; the threshold activity was set at an index of 35 (an 80% increase relative to baseline). The duration of an estrus episode was calculated as the amount of consecutive time whereby the index level was greater than 35. Loss of BW from 2 wk prepartum until 5 wk postpartum was determined for all cows and used to retrospectively group them: (1) those that lost less than or equal to 100 kg of BW (Low; n = 46), and (2) BW loss more than 100 kg (High; n = 21). Data were analyzed by ANOVA using the MIXED procedure of SAS. Increased BW loss was associated with decreased estrous expression as measured with an AAM. Animals within the High group had decreased peak activity (63.6 ± 3.0 vs. 74.0 ± 3.9 peak activity index; P = 0.04) and decreased duration of estrus (8.4 ± 0.7 vs. 11.5 ± 0.9 h; P < 0.01). Ovulatory follicle diameter and estradiol concentration in plasma were unaffected. Additionally, cow within the High group had increased BHBA concentrations (0.57 ± 0.12 and 0.90 ± 0.10 mmol/L; P = 0.03) and reduced glucose concentrations (48.3 ± 1.5 vs. 53.8 ± 1.6 mmol/L; P = 0.02) at 2 wk postpartum. Also, the High group had increased days open in comparison to the Low group (163 ± 13 vs. 113 ± 17 d open; P = 0.03). In conclusion, excessive BW loss early postpartum has negative effects on estrous expression as measured by an AAM and is associated with increased days open.