The objective of this study was to characterize the relationship between rumination and subclinical ketosis (SCK) in transition dairy cows. A study was conducted on 4 commercial dairy farms in Eastern Ontario, Canada. A total of 339 dairy cows (107 primiparous and 232 multiparous) were monitored for rumination activity and SCK from 14d before calving until 28d after calving. Rumination was recorded daily using an automated monitoring system. A blood sample was taken from the coccygeal vein of each cow for the measurement of β-hydroxybutyrate (BHBA) 1x/wk throughout the 6-wk observation period. Cows with a BHBA concentration ≥ 1.2mmol/L postpartum were considered to have SCK. Cases of retained placenta, metritis, milk fever, or mastitis during the study period were also recorded. Cows were categorized into 1 of 3 groups: healthy (H) cows had no SCK or any other health issue (n = 139); SCK (K) cows with no other health problems during transition (n = 97); or ketotic plus (K+) cows that had SCK and one or more other health problems (n = 53). Data were summarized by wk and analyzed in a repeated measures general linear mixed model. From 2 wk before (−2) calving to 4 wk after calving (+4), there was no difference in daily rumination time (409 ± 9.8 min/d; mean ± SE) among H, K, and K+ cows in their first lactation (P = 0.5). Multiparous cows in H spent an average of 459 min/d ruminating from wk −2 to wk +4. Multiparous K cows ruminated 25 ± 12.8 min/d less (P = 0.05) than H cows, while K+ cows ruminated 44 ± 15.6 min/d less (P = 0.005) than H cows. The largest differences in rumination time between H and K+ cows were seen during wk −1, +1 and +2, when K+ cows ruminated 48 ± 17.2 min/d, 73 ± 16.0 min/d, and 65 ± 19.4 min/d less (P ≤ 0.005) than H cows, respectively. These results suggest that rumination monitoring across the transition period might contribute to identification of SCK and other health issues in multiparous cows.