Introduction - Clinicians managing NSIP (non specific interstitial pneumonitis) face a treatment dilemma whilst waiting for the clinical course of the disease to declare itself as IPF (idiopathic pulmonary fibrosis) or other forms of ILD (interstitial lung disease) and the timing of initiating anti fibrotic medication. Aims - 18F-FDG uptake in background lung activity (SUVmin) measured by PET (position emission tomography) in NSIP may predict mortality. A correlation between PET parameters and response to immunosuppressive treatment with cyclophosphamide may solidify its use as a response biomarker in NSIP. Methods - 96 patients with NSIP were prospectively recruited for 18F-FDG PET/CT and had lung function at baseline. 13 patients with a diagnosis of NSIP treated with cyclophosphamide and their pre- and post-treatment change in pulmonary function tests (PFTs) and 18F-FDG PET parameters were analysed. Results - Mortality was associated with high pulmonary SUVmin (p=0.001). In the group treated with cyclophosphamide, TBR (target to background ratio SUVmax/SUVmin) was significantly higher (p=0.046) in post-treatment (mean TBR 6.72) compared to pre-treatment (mean TBR 4.64) in the IPF sub-group. Discussion - In the group of patients who were treated with cyclophosphamide, the increase in TBR post-treatment (compared to pre-treatment) could be attributed to the IPF patients not responding as favourably to immunosuppressive treatment highlighting those that may benefit from earlier initiation of anti-fibrotic treatment.