Objectives were to evaluate: 1) the ability of a commercial rumination (Rum) and activity (Act) monitoring system (HR Tags, SCR Dairy) to identify cows with health disorders and 2) the interval between the day of diagnosis of disease and day of alert by the HR system (HR). Holstein cows (n = 1,118; 449 nulliparous and 669 multiparous) were fitted with an HR tag from −28 to 80 DIM. Every 12 h after 1 DIM, an individual cow Health Index (HI) was generated based on Rum and Act. Cows with a HI value < 86 points were flagged by HR. Farm personnel examined cows for signs of clinical disease (CDZ) daily. From 1 to 10 DIM, personnel evaluated: appetite, rectal temperature, ketone bodies in urine, rumen fill and movements, vaginal discharge, daily milk weights, and conductivity. Data from 1,099 cows was available. Number of CDZ events included was: displaced abomasum (DA) 41, ketosis (KET) 57, indigestion (IND) 9, metritis (MET) 360, and mastitis (MAST) 74. Sensitivity (Se) of HR to flag cows with CDZ (farm personnel diagnosis as gold standard) and the interval between day of CDZ diagnosis and the day a cow was flagged by HR was evaluated with PROC FREQ and PROC TTEST of SAS, respectively. The Se of HI was: 97.6% (CI 93–100%) for DA, 84.2% (CI 75–94%) for KET, 88.9% (CI 68–100%) for IND, 45.6% (CI 40–51%) for MET and 49.7% (CI 42–57%) for all MAST. For all DA, KET, and IND combined, Se of HR was 89.7% (CI 84–95%). Sensitivity of HR by MAST pathogen was 66.7% (CI 50–83%) for E. coli and Klebsiella, 52.6% (CI 37–68%) for Streptococcus and Staphylococcus spp. combined, and 22.2% (CI 0–49%) for Staph. aureus. Mean and 95% CI for interval between day of CDZ to day flagged by HR (cows flagged only) was: −3 (−3.7- −2.3; P < 0.01), −1.6 (−2.3- −1.0; P < 0.01), −0.5 (−1.5- 0.5; P = 0.28), −0.8 (−1.2- −0.44; P < 0.01), −0.8 (−1.2- 0.3; P < 0.01), for DA, KET, IND, MET, and all MAST, respectively. We conclude that the HR system is most effective to identify cows suffering metabolic and digestive disorders. A relatively lower Se to identify cows with MET and MAST might be explained by less severe systemic illness and type of mastitis-causing pathogen. The HR system identified cows with DA, KET, MET and MAST earlier than farm personnel.