BACKGROUND: Mosaic attenuation on CT has been identified in international guidelines as an important diagnostic feature of fibrotic hypersensitivity pneumonitis (FHP) as opposed to idiopathic pulmonary fibrosis (IPF). However, mosaic attenuation (MA) comprises several different radiological signs (low density lobules [LDL], preserved lobules [PL], air trapping [AT] and the so-called "headcheese sign") which may have differing diagnostic utility. Furthermore, the extent of MA required to distinguish these two diagnoses is uncertain, and thresholds of MA from international guidelines have not been validated. METHODS: Inspiratory and expiratory CTs were evaluated by two readers in 102 patients (IPF n=57; FHP n=45) using a semiquantitative scoring system for MA. Findings were validated in an external cohort from a secondary referral institution (IPF n=34; FHP n=28). RESULTS: LDL and AT were a frequent finding in IPF, present in up to 51% of patients. A requirement for increasing extent of LDL and AT based on guidelines (ATS and Fleischner Society) was associated with increased specificity for the diagnosis of FHP (0.96 and 0.98, respectively) but reduced sensitivity (0.16 and 0.20 respectively). The "headcheese" sign was found to be highly specific (0.93), and moderately sensitive (0.49) for a high confidence diagnosis of FHP. The high specificity of the headcheese sign was maintained in the validation cohort, and when patients with other CT features of FHP were excluded. CONCLUSION: MA is a frequent finding in IPF. However, the headcheese sign can be confidently considered as being inconsistent with a diagnosis of IPF and specific for FHP.