BACKGROUND Influenza and respiratory syncytial virus (RSV) infections cause seasonal excess mortality and hospitalisation in adults (particularly the elderly) in high-income countries. Little information exists on the impact of these infections on adults in Africa. OBJECTIVE To estimate influenza- and RSV-related adult mortality, stratified by age and hospitalisation in Soweto. METHODS A retrospective hospital-based study in Soweto from 1997 to 1999 to estimate influenza- and RSV-related excess all-cause deaths and hospitalisation using a rate-difference method. The study was based on influenza seasons of varying severity, provided by surveillance data. RESULTS Influenza seasons were significantly associated with excess mortality in adults across all 3 years, except for 18 - 64-year-olds in 1998. Excess mortality was highest in those > or = 65 years of age: 82.8/100 000 population in the mild 1997 season and 220.9/100 000 in the severe 1998 season. Influenza significantly increased adult medical hospitalisation in the severe 1998 season alone. RSV did not significantly affect mortality or hospitalisation. CONCLUSIONS Influenza-related mortality was substantial and disproportionately affected the elderly. Influenza vaccination for the elderly warrants consideration. The RSV-related burden was not significantly increased but merits observation over a longer period.