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The aim of this study was to investigate the effect of the temperature-humidity index (THI) and days in milk (DIM) on milk production traits and somatic cell score (SCS) of dairy cows raised in north area of Iran. To calculate THI, ambient temperature and relative humidity were obtained from the nearest stands. Milk production data included 67,774 test-day records for milk, fat, and protein yield, fat, and protein percentage and SCS. These traits were associated with the average THI of the 3-d preceding the respective measurement, which was divided into five classes, 40-50, 50-60, 60-70, 70-80, and ≥80). Greatest milk yields were recorded in THI≤60 (P<0.05). The highest decrease in milk yield in connection with THI values were recorded in the early lactation (0 to 100 DIM). SCS was positively associated with the THI and increased more in early period of lactation.