FK506 is a macrolide immunosuppressant agent used in solid organ and bone marrow transplantation and for autoimmune disorders. FK506 is reported to have a number of neuropsychiatric side effects, including anxiety and tremor. Because FK506 was implicated in causing akathisia in a case report, we did a prospective, cross-sectional study of 25 renal transplant recipients to determine whether akathisia occurred and/or had a relationship to FK506 plasma levels. The Symptom Checklist-90-R, Hamilton Anxiety (HAM-A), and Akathisia Rating (ARS) scales were administered. Higher FK506 plasma levels correlated with higher HAM-A scores. ARS scores did not correlate with FK506 plasma levels; however, when FK506 plasma levels were divided into "high" (> or = 0.9 ng/mL) and "low" (< 0.9 ng/mL) groups, total ARS and HAM-A scores were significantly higher in the "high" group. We discuss implications of these findings as well as management.