Poor reproductive performance is a major problem on dairy farms across Northern Ireland and the United Kingdom (UK) as a whole, and recent studies suggest that the problem is increasing. In order to identify the key factors influencing reproductive performance at farm level, a major research initiative was established in 1998 with the objective of collating a comprehensive database on reproductive performance from 20 herds, representing over 2000 dairy cows, across Northern Ireland. Preliminary results for five herds from the first year of the study indicate a high heat detection rate (84%) across all farms. The mean intervals to first observed heat and to first service for cows calved within 19 days of the start of the breeding season were similar between herds (42.8 ± 14.0 days and 50.0 ± 11.0 days respectively). In contrast, the interval to first service for all recorded services was 89.8 ± 66.3 days and varied considerably between farms (69.9 to 112.7 days). Investigation of progesterone profiles indicates that 62.3% of cows had resumed normal cyclic activity by 40 days post calving. Of the cows with atypical ovarian patterns, 19.4% were classified as prolonged post-partum anoestrus with a further 12% exhibiting at least one post-partum prolonged luteal phase.