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We conducted a retrospective cohort study to investigate factors influencing the reproductive performance of dairy cattle in Khorosan Razavi province, Iran. Ten dairy herds, located within a 45 kilometre radius of the city of Mashhad in Khorosan Razavi province took part in the study. Complete lactation records were collected for cows that calved between 21 March 2006 and 20 March 2007. Each cow was followed until the end of the study on 21 October 2007 or until the date of leaving the herd, either by culling, sale, or death. Median days open was 123 days (range 28 to 430 days). Parametric accelerated failure time models based on the log-normal distribution, with and without a herd-level frailty term were used to identify and quantify the effect of factors influencing days open. Parity and the presence of retained fetal membranes, uterine infection, cystic ovarian disease, mastitis and lameness were positively …