Lactation persistency is defined as the ability of a cow to maintain production at a higher level after peak yield. Hypothetically, more persistent cows are less susceptible to health and reproductive disorders. The objective of this research was to investigate the phenotypic relationships of common health disorders in dairy cows to lactation persistency. The relationships with peak yield and days in milk (DIM) at peak yield were also studied. Two separate investigations (Study 1 and Study 2) were performed. Study 1 used treatment incidence data and daily milk weights of 991 lactations from experimental dairy herds at Virginia Tech and Pennsylvania State University. Milk yield persistency (PM) was estimated for individual lactations using daily milk weights. In Study 2, producer recorded health data of 87555 lactations from 398 commercial herds were used. PM, fat (PF), and protein (PP) persistencies were estimated from TD yields. Mastitis only in the first 100 days, only after 100 DIM, and at any stage of lactation, and reproductive disorders including metritis, retained placenta, cystic ovaries, metabolic diseases including ketosis, milk fever and displaced abomasums, and lameness were considered in both studies. Mastitis both before and after 100 DIM was considered only in Study 1. Each disease was defined as a binary trait distinguishing between lactations with at least one incidence (1) and lactations with no incidences (0). Standardized measures of the persistencies, uncorrelated with yield, were calculated as a function of yield deviations from standard curves and DIM deviations around reference dates. Peak milk yield and DIM at peak of individual lactations were computed using Wood's function. Effects of persistency (PM, PF, and PP) on probability of the diseases in current and next lactations were examined through odds ratios from a logistic regression model. Conversely, the effects of diseases on persistencies, peak milk yield, and DIM at peak milk yield were also examined. Increasing PM, PF, and PP tend to reduce the incidence of mastitis, specifically in late stages of current and next lactation. PM and PP appear to have greater impact on mastitis than PF. No other likelihood of a disease was affected by the increasing persistencies. Post partum reproductive and metabolic diseases often had substantially positive effect on persistencies of both primiparous and multiparous cows (p<0.001 in Study 1 and p<0.001 in Study 2). Mastitis in early lactation appeared to increase persistency more often in multiparous cows (p<0.05 in Study 1 and p<0.005 in Study 2). Mastitis in late lactation had considerable but negative impact on persistency in both primiparous and multiparous cows (p<0.05 in Study 1 and p<0.005 in Study 2). Cows, which developed mastitis in both early and late lactations tended to have lower PM (p<0.05 in Study 1). Irrespective to the time of occurrence, effect of mastitis on milk, fat and protein yield persistencies was negative. Most of the diseases significantly affected DIM at peak milk yield in multiparous cows (p<0.05 in Study 1). Reproductive and metabolic disorders tended to delay DIM at peak milk yield while Mastitis in late lactation was associated with early DIM at peak milk yield. Lameness had no phenotypic relationships with shape of the lactation curve. Overall, diseases tend to affect milk, fat, and protein persistencies more strongly than the impact of persistency on likelihood of disease. / Master of Science