Seven cases of crystalluria following primidone overdose have been reported since the 1950s. An eighth case of primidone crystalluria following overdose is presented. Because of low aqueous solubility (600 mg/L at 37 degrees C) which is directly proportional to temperature, any factor increasing renal excretion of unchanged primidone predisposes to crystal formation. Renal clearance is dependent on dosage because of negligible protein binding, zero-order conversion to phenobarbitone (phenobarbital) and first-order conversion to phenylethylmalonamide. Therapy with other anticonvulsants known to induce the metabolism to phenobarbitone does not appear to be protective against crystalluria in overdose situations. The critical serum primidone concentration for crystalluria presence seems to be 80 mg/L. There is evidence for nephrotoxicity of the crystals themselves if formed in vivo (actual crystal presence during voiding). The chemical phenomenon of supersaturation of a solution is protective against in vivo crystal formation with subsequent nephrotoxicity. Vigorous hydration to augment elimination and to lessen the propensity for renal toxicity is recommended.