Over the past decade it has been realized that HIV affects the central nervous system, and various investigations have illuminated the spectrum of neuropathology in AIDS. One major advance has been the demonstration that there is substantial neuronal loss, which appears independent of the HIV-associated inflammatory lesions. Quantitative studies on neuronal populations, while fraught with methodological difficulties, are essential to the understanding of the mechanism of this neurotoxic damage. This article will review, firstly, the modern stereological procedures available for quantitative investigations; secondly, the pattern, degree and time scale of HIV-associated neuronal loss; thirdly, other morphological evidence of neuronal damage; and finally, the pathological and clinical implications of these findings.