 Specific Aim 1: To test the hypothesis that emphysema is more prevalent and progresses more quickly in HIV subjects compared to HIV- controls: The investigators will characterize the prevalence, nature, and progression of HIV-associated emphysema. Emphysema will be determined in HIV and HIV- subjects by spirometry, diffusing capacity, and quantitative computed tomography andor EBCT. These studies will be repeated at 18 and 36 months after baseline to assess differences in progression between HIV and HIV- subjects. Multifactorial regression analyses will determine the contribution of various risk factors to presence and progression of emphysema. 