The aim of the present study was to estimate the genetic parameters and genetic trends of somatic cell score (SCS) in the first 3 lactations of Iranian Holstein cows by random regression (RR) animal model by using Restricted Maximum Likelihood (REML) method. The data set used in this analysis included observations of 340318 test-day records of 41526 cows in 288 herds; 89969 test-day of 11750 cows in 127 herds and 20010 test-day of 2461 cows in 60 herds in the first-, second- and third-parity, respectively, during the period from 2002 to 2010. Fixed effects considered were: year of calving, age-season of calving (by applying a fixed regression for each subclass of this effect) and herd-test date. Estimates of additive genetic variances for all parities, with only small changes, show a similar trend. Heritability estimates for first parity (0.03 to 0.07) were lower than those for second (0.07 to 0.11) and third (0.08 to 0.17) parities. Genetic correlations between daily SCS within parity were high for adjacent tests and low between the beginning and the end of lactation and decreased with increase parity number. Genetic correlations between parities were higher than 0.70 but environmental correlations between parities were in range of 0.22 to 0.51. Genetic correlations between parity demonstrate that genes that control SCS in parities are same and selection animal for SCS in first parity would have affected SCS in other parity. The genetic trend for SCS was favorable until 1995 and unfavorable since then. The relatively high level of SCS and positive genetic trends (0.431 to 0.701 cells/mL/year) would suggest that genetic improvement of SCS is at unacceptable level in Iran. Manuscript profile