A goat herd severely affected by arthritis was studied. The most representative clinical signs consisted of articular swelling, mainly of the carpal joints, and the subsequent locomotor disorders. Some goats also showed signs of central nervous system involvement. Examinations of joint fluid revealed an increased number of mononuclear blood cells, mostly lymphocytes. Gross and microscopic articular lesions were of inflammatory and degenerative types. Periarticular connective tissue, synovial bursae, tendons and tendon sheaths were predominantly affected. Inflammatory lesions were those of a chronic hyperplastic tenosynovitis with fibrosis of the connective tissue components. Degenerative changes consisted mainly of necrosis and mineralisation of articular-related structures. Histological lesions in the central nervous system were those of a nonpurulent encephalitis initially located in periventricular areas, but in one case extensive encephalomalacia was also seen. Of the 80 animals sampled 82.5 per cent showed seropositive reactions against an ovine progressive pneumonia virus antigen. None was seropositive to brucella and titres to chlamydia were low. Attempts to isolate chlamydia and mycoplasma from affected joints and several organs failed. Different bacteria were recovered from a few samples but did not seem significant. Syncytium-forming viral particles were isolated from several organs, mainly the lungs, synovial membranes and lymphoid tissue of almost all the slaughtered animals. These particles were identified as lentiviruses by electron microscopy. The clinical signs, lesions serological results and microbiological findings, led to a diagnosis of caprine arthritis-encephalitis. This syndrome has not been recognised in Spain previously.