Purpose: To prospectively evaluate, by using positron emission tomography (PET) and magnetic resonance (MR) imaging, the interrelationships between regional myocardial fibrosis, perfusion, and contractile function in patients with idiopathic dilated cardiomyopathy (DCM). Materials and Methods: The study protocol was approved by the hospital ethics committee, and all subjects gave written informed consent. Sixteen patients with idiopathic DCM (mean age, 54 years ± 11 [standard deviation]; nine men) and six healthy control subjects (mean age, 28 years ± 2; five men) were examined with PET and MR tissue tagging. Oxygen 15-labeled water and carbon monoxide were used as tracers at PET to assess myocardial blood flow (MBP) and the perfusable tissue index (PTI), which is inversely related to fibrosis. MBF was determined at rest and during pharmacologically induced hyperemia. Maximum circumferential shortening (Ecc) was determined with MR tissue tagging. Student t tests were performed for comparison of data sets, and linear regression was used to investigate the association, between parameters. Results: Mean global hyperemic MBF (2.23 mL/min/mL ± 0.73), Ecc (-10.5% ± 2.9), and PTI (0.95 ± 0.10) were lower in the patients with DCM than in the control subjects (4.33 mL/min/mL ± 0.85, -17.4% ± 0.6, and 1.09 ± 0.12, respectively; P < .05 for all). In the patients with DCM, regional PTI was related to Ecc (r = -0.21, P = .009) but not to resting or hyperemic MBF. Furthermore, regional Ecc was correlated to both resting (r = -0.28, P = .004) and hyperemic MBF (r = -0.29, P < .001). In addition, the ratio of left ventricular end-diastolic volume to mass, as a reflection of wall stress, was related to global hyperemic MBF (r = -0.52, P = .047) and to global Ecc (r = 0.69, P = .003). Conclusion: In idiopathic DCM, the extent of myocardial fibrosis is related to the impairment in contractile function, whereas fibrosis and perfusion do not seem to be interrelated. The degree of impairment of hyperemic myocardial perfusion is related to contractility and end-diastolic wall stress.