Studies were undertaken to characterize virus like particles (VLPs) that have been observed in the feces of psittacine birds with proventricular dilatation disease (PDD). To determine if the VLPs have any association with the disease, fecal samples were collected from 24 PDD and 8 non-PDD birds and were examined by electron microscopy. Fifteen out of 24 PDD birds were shedders of the VLPs. Among the non-PDD birds, 3 of them shed the VLPs. The VLPs measured 80-100 nm in their short diameter (for spherical, oval, elongated and bean shaped VLPs) and 120-160 nm (for bean and elongated shaped VLPs) in their largest diameter. All VLPs had "Y" shaped surface projections of 12.5 (±1.5) nm in length. In addition to the above-described VLPs, incidental small non-enveloped circovirus like particles of 27-35 nm in size were detected in 2 PDD and 1 non-PDD bird. No significant statistical relationship (X2 0.62; p = 0.41) between the presence of the VLPs and PDD was determined. Further attempts to isolate the VLPs in cell culture (CH-SAH, QT-35 and primary cell cultures of fibroblasts, brain and lung cells) and embryonating eggs (psittacine and chicken origin) and to characterize them molecularly were unsuccessful. Necropsies were performed on 34 PDD birds of various species and macroscopic and microscopic lesions were recorded. Macroscopic findings that have not been reported in association with the disease before included hydropericardium, dilatation of the right ventricle of the heart and accumulation of transparent fluid in the subarachnoidal space of the brain. The microscopic lesions in the gastrointestinal tract (GIT), brain, spinal cord, heart and adrenal glands were mainly characterized by lymphoplasmacytic infiltrates as previously reported. Lymphplasmacytic infiltrates were observed in the ventriculus (97%), heart (78%), spinal cord (63%), brain (60%), proventriculus (53%), adrenal glands (50%) and crop (43%). Histological lesions that differed from previous reports of PDD were observed in the peripheral nerves: sciatic (45%), brachial (35%) and vagus (29%), and in the dorsal root ganglia of spinal cord and optic nerve. The correlation between the clinical signs of the disease and presence of microscopic lesions in 24 birds was also studied. There was a strong correlation between the neurological signs and presence of microscopic lesions in the spinal cord (r = 0.389; p = 0.034), sciatic (r = 0.424; p = 0.024) and vagus (r = 0.524; p = 0.007) nerves. There was no significant correlation between the digestive signs and microscopic lesions in the digestive tract. Attempts of virus isolation in cell cultures (Vero, QT-35 and primary cell cultures of fibroblasts, lung and brain cells) and embryonating SPF chicken eggs from tissues of 10 PDD birds were not successful. No viruses were detected in tissue homogenates from the brain, CSF, spinal cord and GIT.