This prospective observational study of positron emission tomography (PET)-MRI findings in 16 consecutive newly diagnosed patients with a plasma cell dyscrasia describes and compares MRI-detected myeloma lesions with F-fludeoxyglucose PET-avid myeloma lesions, and correlates quantitative imaging findings to a range of biochemical and prognostic parameters. Simultaneously acquired whole body PET and MRI images were evaluated qualitatively for the presence of focal or generalised abnormalities of bone marrow (BM) on either modality. Quantitative analysis comprised mean standardised uptake values (SUVmean) and fractional water content of the BM measured from PET and chemical shift MRI images of the second to fourth lumbar vertebrae. Final diagnoses comprised symptomatic myeloma ( = 10), asymptomatic myeloma ( = 4) and monoclonal gammopathy of uncertain significance ( = 2). 8/10 patients with symptomatic myeloma demonstrated BM abnormalities on qualitative assessment of MRI compared to 4/10 on PET. BM SUVmean inversely correlated with serum albumin ( = 0.57, = 0.017). BM water fraction correlated with trephine cellularity and blood platelet count ( = 0.78, = 0.00039 and = 0.61, = 0.0013 respectively). BM water fraction correlated with SUVmean in patients with low plasma cell burden ( = 0.91, = 0.0015) but not in patients with high plasma cell burden ( = 0.18, = 0.61). PET-MRI shows promise in both morphological and functional multiparametric quantitative assessment of myeloma. For the first time, multiparametric imaging in myeloma has been shown to predict BM abnormalities and correlate with known biochemical prognostic markers, moving PET-MRI beyond simple diagnostic applications into potential prognostic and treatment selection applications.