Acute metabolic acidosis associated with accumulation of lactate has been previously reported in isoniazid (INH) intoxication. To our knowledge, association of INH toxicity with beta-hydroxybutyric acidosis has not been demonstrated previously. The present report documents the occurrence of beta-hydroxybutyric acidosis in patients with INH intoxication. The reason for the lack of previous reports of this association is not clear, although failure to measure plasma beta-hydroxybutyrate levels in previous studies is a likely possibility. Our patients received intravenous sodium bicarbonate, anticonvulsants and dialysis which resulted in complete reversal of metabolic acidosis and other manifestations of INH toxicity.