The prevalence of low serum urea nitrogen concentrations (less than 50 mg/L) in our patient population was 1.2% (151 per 12,380 determinations), representing 95 individual cases. Of these, 81 of the patients' charts were located, reviewed, and classified into two groups, those with and those without hepatobiliary disease. Hepatobiliary disease was found in 36% of the 81 patients; 90% of these showed evidence of alcohol abuse, as did 19% of those without hepatobiliary disease. The remaining patients without hepatobiliary disease had various clinical conditions: psychiatric disorders (14.8%), overhydration (12.3%), endocrine disorders (7.4%), cardiovascular diseases (4.9%), prednisone administration (3.7%), and special diets (2.5%). Thus, in our patient population the most frequent cause of low serum urea nitrogen concentrations was alcohol abuse, found in about half of the cases.