Over a 3 week period in January 2011, a 2800 cow dry lot dairy in the Texas panhandle feeding a balanced total mix ration (TMR) with free-choice salt with a daily production of 73 lbs. milk/cow experienced severe, acute neuromuscular signs in 12 high-producing multiparous lactating cows 30-80 days in milk (DIM). Affected cows were found with varying degrees of ataxia and recumbency in their pens and 6 of 12 cows died. Recent changes in the ration included removing a bypass protein and increasing urea, soybean meal, and alfalfa silage in the TMR to reduce the cost of protein precursors in the ration. Milk urea nitrogen (MUN), which was being monitored daily at this dairy, increased from 9 mg/dL to 13.5 mg/dL over the week preceding the onset of clinical signs. Clinical signs of affected cows ranged from ambulatory cows with an abnormal gait marked by paresis and tremors to laterally recumbent cows with combinations of dyspnea, tachycardia, paddling, bruxism, hypersalivation, eructation, and hind limb rigidity. A presumptive diagnosis of hypomagnesemia was made based on information collected from physical exam, response to various treatments (e.g., fluid therapy including lactated ringers solution, calcium-magnesium-phosphorous-potassium-dextrose solution, magnesium sulfate, and ration adjustments to correct for potassium and magnesium concentrations), chemistry panels, vitreous analysis for nitrate/nitrite, rumen ammonia levels and pH, necropsy, and forage analysis.