Approximately 45 Holstein cows that were Mycobacterium paratuberculosis-positive on the basis of fecal culture were maintained at any one time in a 210-cow dairy herd. Paratuberculosis-positive cows were grouped separately from negative cows, but they were otherwise managed identically. During a 1-year study, 180 paratuberculosis-negative cows and 113 clinically normal paratuberculosis-positive cows were identified by fecal culture through the laboratory of the NY State Paratuberculosis Program. Quarter milk samples were asceptically collected for culture of mastitis pathogens: 6,100 milk samples from paratuberculosis-negative cows, and 3,129 quarter samples from paratuberculosis-positive cows. Dairy Herd Improvement Association (DHIA) records were used to monitor milk somatic cell count Linear Score (LS), mature equivalent (ME) milk production, new mastitis infections, and chronic mastitis infections. Beginning in second lactation, and increasing with advancing parity, paratuberculosis-positive cows had lower ME milk production than did negative herdmates. However, rates of new and chronic mastitis infections, as measured by DHIA LS were significantly (P<0.05, P=0.05, respectively) lower in cows with nonclinical paratuberculosis. Infected cows were culled from the herd at a faster rate than were negative herdmates. Overall, paratuberculosis was associated with economic benefit due to lower rates of mastitis in positive cows, but a net financial loss resulted because of reduced milk production and increased culling rates.