Abstract. Calving records from the Animal Breeding Center of Iran collected from January 1986 to December 2007 and comprising 492424 calving events of Holsteins from 1413 dairy herds were analysed using linear animal models to estimate genetic parameters for lactation lengths in the first three lactations of Holstein cows. Genetic trends were obtained by regressing yearly mean estimates of breeding values on calving year. Heritability estimates for lactation lengths varied from 0.03 to 0.04 across the parities. Estimates of genetic correlations between lactation lengths in different parities ranged from 0.35 to 0.53. There was decreasing genetic trend for lactation length at parity 3 and there were increasing genetic trends for lactation lengths at parities 1 and 2 over the years (P<0.05). The low estimates of heritabilities obtained in this study imply that much of the improvement in lactation lengths could be attained by improvement of production environment rather than genetic selection.