Background: Pulmonary fibrosis (PF) is a tissue repair mechanism that can be abnormally dysregulated by ageing, predisposing environmental factors and microbes. It is poorly documented in horses. An association between PF disorders and y-herpesvirus infection has been established in several species. Objectives: This study aims to establish a histological fibrosis score (HFS) system on lung histological sections and to assess the potential relationship with age, EHV-5 viral load in lung tissue and clinical outcome. Study design: Retrospective clinical study. Methods: Clinical information from horses presented for respiratory distress and interstitial lung diseases (ILD) was recorded. Antemortem transthoracic lung biopsies or post-mortem pulmonary samples from horses with severe generalised interstitial radiographic patterns, and previous histological identification of replacement of alveoli by fibrosis and collagen were selected. The severity of PF was assessed blindly using a previously described histologic scoring (/8) [1]. EHV-5 qPCR (Ct) was performed on lung tissue. Shapiro–Wilk, Fisher and Spearman correlation tests were used to analyse the data. Results: Twenty-five cases were included. Median age was 14-yo (4–29-yo). EHV-5 was detected in 12 of 25 horses (48%). HFS ranged from 1 to 6/8 (median 4/8) and was not different between EHV-5-or negative groups (p = 0.7). Eighteen out of 25 horses (72%) and 12/25 (50%) with PF survived to discharge and 3 months, respectively, with no statistical difference between EHV-5 positive and negative horses (p = 0.7). HFS was moderately negatively correlated to short-term survival (r = 0.55, p = 0.005) but not survival at 3 months (r = -0.37, p = 0.07). HFS was not statistically correlated to age (r = -0.15, p = 0.4), nor Ct (r = - 0.07, p = 0.7). Main limitations: Small number of cases, retrospective nature of the analysis. Conclusions: EHV- 5 is a common cause of PF in horses. Age, EHV-5 viral load, and severity of PF do not seem to influence survival rate of equines affected by fibrosing ILD. Ethical animal research: Approved by VetAgro sup Ethic Committee (n°2040). Informed consent: Owners gave consent for their animals' inclusion. Competing interests: None declared. Funding: None. Reference: [1] Hübner RH, Gitter W, El Mokhtari NE, Mathiak M, Both M, Bolte H, Freitag-Wolf S, Bewig B. Standardized quantification of pulmonary fibrosis in histological samples. Biotechniques. 2008;44(4):507–511. doi 10.2144/000112729.