Dangerous pseudohyponatremia can occur in patients with hyperlipemia. This condition can lead to a misdiagnosis of either hyponatremia or normonatremia. Correction for the effects of lipemia by measuring lipids or ultracentrifuging specimens is clearly not practical in the acute setting. The recommendation of following plasma osmolality, urinary electrolytes, or both is prudent but is not directly addressed by our study. Rather, our study indicates that the pseudohyponatremia is totally dependent on the technique used for measuring sodium. The direct potentiometric techniques that are being increasingly used are not influenced by the presence of lipids, but flame photometric and indirect potentiometric methods give low results. Therefore, it is essential that the laboratory report indicate and the clinician consider which technique was used in measuring the sodium or potassium value in hyperlipemic subjects to correctly interpret the results. This is especially important if both techniques are in use at the same laboratory.