Pituitary pars intermedia dysfunction (PPID) is an age-related disease considered to have a negative impact on fertility. To understand the true impact of PPID on fertility, the influence of age must be considered.This study aimed to assess the impact of PPID on fertility while accounting for the effect of aging.A total of 332 Thoroughbred mares aged 10-20 years and managed across 137 private farms in Hidaka, Hokkaido, Japan were classified as PPID, equivocal, or non-PPID based on plasma adrenocorticotropic hormone concentration in 2019 and 2020. The prevalence of PPID and its associations with age and blood biochemistry parameters, including prolactin, were investigated. Multivariable logistic regression analyses, adjustments for potentially confounders including mare's age, PPID status were conducted to evaluate their impact on fertility.The prevalence of PPID was 9.04 %. No significant differences in prolactin concentrations were observed among PPID status groups. The adjusted odds ratio (AOR) for the interaction between age and PPID status was 0.49 (95 %CI; 0.26-0.91, p = 0.02), indicating a greater age-related decline in pregnancy rate in PPID mares compared to non-PPID mares. Of the 30 PPID mares, 24 were assigned to either treated or untreated groups, and monitored for pregnancy outcomes in the following spring. Pregnancy rates were 80 % (12/15) in treated mares and 44.4 % (4/9) in untreated mares (p = 0.10). The AOR for pregnancy in treated mares, relative to untreated mares, was 2.98 (95 %CI; 0.26-24.61, p = 0.31).PPID was suggested to negatively impact on fertility with aging.