A 5-year-old Chinese girl had had absence seizures and received sodium valproate (VPA) treatment which provided good control. Six months later, she developed interstitial nephritis with proteinuria and microhematuria. Renal biopsy revealed interstitial nephritis with granular deposition of immunoglobulin G (IgG) and C3 in the renal tubular basement membrane (TBM). Ultrastructurally, dilated smooth endoreticular cisternae with mitochondrial degeneration in the tubular cells and scattered electron-dense deposits within the TBM were also noted. Serum circulating immune complexes were detectable, ACH50 and properdin factor B increased. Mononuclear cells (MNC) from the patient after in vitro incubation with VPA (100 micrograms/ml) induced interleukin-2 (IL-2) production and lymphoproliferative response. However, there was no response in controls. The serum VPA level ranged from 84 to 92 micrograms/ml. After VPA was stopped, the microhematuria and proteinuria disappeared. These observations indicate that VPA-induced interstitial nephritis represents a sequence of interrelationships among multiple immunologic factors.