The cost of milk production by farm size was decomposed into frontier and efficiency components with a stochastic cost curve using data on 755 USA dairy farms from the year 2000. The estimated frontier function is much flatter than the composite cost curve, and although the frontier cost of production decreases with farm size, that cost reduction is not as pronounced as a cost curve that includes inefficiency. The higher cost of production of many smaller farms is caused by inefficiency. The 50-cow farm has a frontier cost of production of $10.05 and an inefficiency cost of $10.27 for a composite cost of $20.32. In contrast, the 1,000-cow herd has a frontier cost of production of $9.27 and an inefficiency cost of $2.82 for a composite cost of $12.09. The implication is that the efficient 50-cow farm is competitive with the average 1,000-cow farm, but not with the efficient 1,000-cow farm.