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The phenomenon of geometric mispositioning (defined as having a geometric proper plus a parallax component) was studied and characterized on Neuro- and whole body PET systems employing 2-D modular detectors (CTI/Siemens ECAT-831/08 and 931/08) as well as an older system (CTI ECAT-911). Measurements taken with precisely spaced line sources showed image distortions of objects away from the center of the field of view (FOV), with global mispositionings of -1.4, 1.5 and 14 mm (931 and 911), and -0.5, 10 mm (831) at 10, 20 and 30 cm (931 and 911) and 10, 20 cm (831) from the center of the FOV. Structures as close as 5 cm (931/911) or 4 cm (831) to the center of the FOV are mispositioned by more than 1 mm. This is clearly inacceptable in cases where accurate correlation of PET and NMR images is needed to acquire anatomical information in neurological studies, or when gated cardiac studies are performed in order to precisely determine myocardial wall thickness. A fast, on-line geometric correction was performed by creating new, uniformly spaced sinograms to be mapped to the original sinograms. Geometric mispositioning was mathematically derived, while parallax mispositioning was estimated by a linear fit of the data partially corrected for sampling non-uniformity. Our correction technique can be easily tailored to any PET system available on the market.