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Swelling of the mammary gland is an important sign to detect clinical mastitis (CM) in dairy cows. The overall objective of this study was to evaluate if udder firmness can be used as a cow-side indicator for mastitis and to evaluate how CM affects firmness within 14 d after diagnosis. A dynamometer was used to objectively determine udder firmness before and after milking in 45 cows with CM and 95 healthy cows. Udder firmness of both hind quarters was measured daily on 3 locations (upper, middle, lower measuring point) from the day of mastitis diagnosis until d 7 and again on d 14. Firmness of the middle measuring point was highest before and after milking in all cows. Udder firmness before milking was similar in quarters without and with CM. Subsequently, we concentrated on firmness measured on the middle point after milking. After milking, quarters with CM were firmer than healthy quarters. An increase of firmness of a quarter with mastitis did not affect firmness of the healthy neighboring quarter, nor did firmness of all healthy quarters differ. One firmness value per cow [i.e., Δ firmness (difference in udder firmness between both hind quarters)] was used for all further calculations. After fitting a generalized mixed model, CM affected Δ firmness in all cases. In multiparous cows, Δ firmness was also affected by continuous milk yield per day and DIM. Firmness thresholds for detection of CM were calculated using receiver operation characteristic curves. The threshold for detection of CM using Δ firmness was 0.282 kg (area under the curve: 0.722, sensitivity: 64.3%, specificity: 89.7%) and 0.425 kg (area under the curve: 0.817, sensitivity: 62.5%, specificity: 96.7%) in primiparous cows and multiparous cows, respectively. Linear mixed-model ANOVA were used to evaluate how CM affects udder firmness within 14 d after diagnosis. Cows with CM had a higher Δ firmness compared with cows without CM throughout the 14 d after the mastitis diagnoses. Parity had an effect on Δ firmness. Depending on systemic signs of sickness, mastitic cows were divided into cows having mild to moderate (n = 21) or severe mastitis (n = 24). Cows with severe mastitis suffered from a firmer udder on all measuring days. Bacteriological cure was defined based on 2 milk samples taken at 7 and 14 d after enrollment. An effect of parity and bacteriological cure on Δ firmness of cows with CM did not exist within the 14 d. Cows not clinically cured showed an increased Δ firmness of 0.560 kg compared with cured cows. In conclusion, udder firmness can be a useful indicator for CM. Further research is warranted to evaluate if udder firmness can be used as a predictor for the prognosis of a CM or the cure of inflammation.