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The plant piquillin (Condalia microphylla) grows in arid regions of Argentina and is the cause of mal del piquillin in cattle. The salient histologic features of this leukomyelopathy are vacuolation of white matter and axonal degeneration in the spinal cord. The histologic lesions can be experimentally reproduced in cattle and pigs by feeding milled bark of the plant, but naturally occurring piquillin toxicosis has not been reported previously in pigs. The authors report an outbreak of progressive ataxia on an Argentine hog farm, where partially consumed piquillin plants were found in the pens of affected pigs. Histologic lesions included vacuolation of white matter, edema, and mild axonal degeneration in lumbosacral segments of the cord. The diagnosis of mal del piquillin was based on the history, clinical signs, histologic changes, and elimination of other potential causes of leukomyelopathy. No new outbreaks developed after elimination of piquillin from the hog lots. Results of this investigation indicate that C microphylla toxicosis can affect pigs under natural conditions and should be included in the differential diagnosis for porcine ataxia and leukomyelopathy in regions where the plant grows.