Bovine paratuberculosis (Johne's Disease) is a chronic debilitating disease of cattle. The disease is widely distributed and dairy veterinary practitioners and dairy producers are becoming aware of the benefits of establishing Johne's Disease control measures. Although there is no doubt that the clinical disease can cause significant economic losses, the sub-clinical consequences of the disease are not well documented. The objective of this investigation was to evaluate the production impact of sub-clinical manifestations of Johne's Disease in dairy cattle based on sero-conversion using an ELISA test. To accomplish this objective, a retrospective cohort study in a commercial dairy located in northwest Texas was conducted. Production data were collected starting April, 2001 using a paired comparison scheme. The cohort consisted of cows of similar age (+/- 30 days), lactation, and origin. Production parameters (lifetime milk production, milk per lactation) and individual performance parameters (days in milk, days in dairy, cull date and reasons for culling) were collected from 120 ELISA positive and 120 ELISA negative for Johne's Disease cows, for as long as the animals were maintained on the premise. Statistical comparison (t-test) and linear regression analysis on the obtained data were conducted. Our results to date indicate a significant reduction in milk production (4,090 kg lifetime milk production per animal) from ELISA positive cows when compared to ELISA negative cows. Also, more ELISA positive cows became lame (5x), developed respiratory disease (1.25x), digestive disease (1.83x), and mastitis (2x) than ELISA negative cows. Currently, 28.33 % of the ELISA positive and 60.34 % of the ELISA negative animals remained in the herd. The reduction of milk production in the ELISA positive cow started in the second lactation and remained for the rest of her productive life, regardless of when the cow was detected as ELISA positive.