Four patients admitted for ethylene glycol intoxication had massive crystalluria, predominantly of needle-shaped calcium oxalate monohydrate--sometimes referred to as the 'hemp seed' habit. A few 'envelope' type crystals of calcium oxalate dihydrate were observed in one urine sample. Although not a pathognomonic sign, the presence of a large number of calcium oxalate monohydrate crystals (Whewellite) supports the diagnosis of ethylene glycol poisoning in patients having a metabolic acidosis of unknown aetiology. We conclude that the monohydrate form rather than the dihydrate form is the predominant type of calcium oxalate crystal formed in ethylene glycol poisoning.