Valproic acid is an anticonvulsant drug known to inhibit the glucuronidation of zidovudine (AZT) in human liver microsomes. Zidovudine is metabolized by glucuronidation to the inactive 5′-glucuronide with a short plasma half-life (1.0 ± 0.2 hour). This case presentation confirms that valproic acid inhibits glucuronidation in vivo, and this is the first documented observation of increased cerebrospinal fluid levels of zidovudine because of an interaction with valproic acid in a patient with acquired immune deficiency syndrome (AIDS). The peak plasma AZT level for the control period was 119 ng/mL, which increased almost 3-fold to 344 ng/mL with valproic acid (1.5 g/day). The plasma AZT trough was 47 ng/mL, which also increased almost 3-fold to 124 ng/mL with valproic acid. The molar ratio of plasma 5′-glucuronide/AZT at the peak was reduced from 1.77 (control) to 1.07 with valproic acid. The 5′-glucuronide/AZT ratio at the trough was reduced markedly from 5.0 (control) to 0.93 with valproic acid, suggesting in vivo inhibition of glucuronidation. Cerebrospinal AZT levels, drawn 30 minutes after peak plasma levels, increased from 27 ng/mL for the control to 47 ng/mL with valproic acid, which paralleled the change in peak plasma concentrations. This interaction with valproic acid may contribute to higher AZT levels in the brains of patients with human immunodeficiency virus-related (HIV) encephalopathy.