There has been a great deal of debate on the impact of increasing performance on the health of food animals. Specifically, in the dairy industry, this performance has been implicated for increasing disease, by both the industry and the animal welfare community. There is overwhelming evidence that the performance of dairy cows has increased over time. However, a summary of the scientific evidence for an association with disease incidence results in a wide range of study findings. The mixed result from grouping studies together is probably due to variation in study design and lack of definitive associations. There is also a wide range of geographic and agricultural systems, making comparisons among reports unreliable. The few studies conducted in our conventional North American agricultural systems indicate that most metabolic diseases of dairy cattle probably have no association with previous milk yield. This information indicates that while disease does impact performance, the reverse cannot be scientifically proven at this time. It also illustrates that perceptions of association can be very different from scientifically proven causation.