The Organic-Conventional Comparative Dairy Systems trial at Massey University began in August 2001, and the organic farmlet achieved certification in August 2003. The trial is unique because it is the only comparative grassland-based open grazing dairy study in the world. The organic and conventional systems are managed individually according to best practice, and both are intensively monitored for production, animal health, and environmental impacts. The systems remained similar for the first two years, but began to diverge in the third and fourth years. Production has been 10-20% lower on the organic farm, but environmental impacts appear to be less than on the conventional unit, and net incomes would be similar given a 20% price premium for the organic product. Animal health issues have been manageable on the organic farmlet, and not too dissimilar from the conventional farmlet. Full results after four years of the trial will be available and presented at the conference