Purpose. The aim of this study was to evaluate the function of the neurosensory retina in Best vitelliform macular dystrophy (BMD) by recording the focal electroretinogram (FERG) fundamental and 2nd harmonic components, which are known to be dominated by receptoral and postreceptoral activity, respectively.Methods. FERGs were recorded in response to a uniform field (9×9 deg) flickered sinusoidally at either 8 Hz or 32 Hz (peak frequencies for the 2nd and fundamental harmonic, respectively). The fundamental component of the response to the 32-Hz stimulus and the 2nd harmonic of the response to the 8-Hz stimulus were measured in their amplitudes and phases. The fundamental-2nd harmonic amplitude ratio was taken as an index of the relative changes in the FERG components. Eleven patients with BMD and vitelliform stage macular lesions were evaluated. Results were compared with those obtained from 13 patients with Type 2 Stargardt macular dystrophy (STD) according to the Noble and Carr classification, and 29 normal control subjects. Four BMD and four STD patients were also followed electrophysi-ologically over a 48 month period.Results. Compared to controls, BMD patients showed losses of both FERG fundamental and 2nd harmonic amplitudes, and an increase in the fundamental-2nd harmonic ratio. STD patients also showed losses of both fundamental and 2nd harmonic, but the fundamental-2nd harmonic ratio was normal. In BMD patients, but not in those with STD, the fundamental amplitude tended to decrease over the follow-up period.Conclusions. The results indicate that BMD involves neurosensory abnormalities early in the disease process. The increased fundamental-2nd harmonic ratio suggests that a postreceptoral dysfunction may be present in addition to that of photoreceptors. This differs from STD, where losses appear to affect primarily the receptoral retina. Receptoral losses in BMD may progress throughout the medium-term follow-up.