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The appropriateness of all serum phenytoin concentrations measured at this 294-bed, tertiary care institution over a 7-week period were evaluated. Each serum level was determined to be justified or unjustified based on either pharmacokinetic or clinical criteria. A total of 234 serum levels were measured in 58 patients. Thirty-eight (16%) of the levels were justified on a pharmacokinetic basis, whereas 70 (30%) were justified for clinical assessment of the patient. One hundred thirty (56%) of the serum levels could not be justified on any grounds. Also identified were distinct clinical situations for 13 patients for which monitoring of serum levels was warranted but not performed. Most phenytoin serum level measurements at this institution were not reasonably justified and may have been unnecessary. A yearly reduction in laboratory costs in excess of $9,000 was predicted if such serum levels had been ordered only when clearly indicated.