A simple isocratic reversed-phase HPLC method for the determination of gabapentin and its major degradation impurity, 3,3-pentamethylene-4-butyrolactam, was developed and validated for use in the analysis of pharmaceutical tablets and capsules. Separation was achieved on a Brownlee Spheri-5 Cyano column using an acetonitrile-10 mM KH2PO4/10 mM K2HPO4 (pH 6.2) (8:92, v/v) mobile phase. The compounds were eluted isocratically at a flow rate of 1 mL/min. Both compounds were analyzed with UV detection at 210 nm. The method was validated according to USP Category I requirements for gabapentin and USP Category II for 3,3-pentamethylene-4-butyrolactam. The validation characteristics included accuracy, precision, linearity, range, specificity, limit of quantitation and robustness. Validation acceptance criteria were met in all cases. This method was used successfully for the quality assessment of four gabapentin drug products.