We prospectively collected data on deaths in the Edinburgh City Hospital HIV cohort of patients (60-70% acquired via injection drug use) from October 1986 to September 1994. Sixty-four patients (25% of all HIV deaths or 2.5/100 person-years) had died without an AIDS diagnosis, and 42 (66%) of these had autopsy data available. Some pre-AIDS deaths (20% or 0.5/100 person-years) were the expected consequence of underlying medical conditions diagnosed during life: the remainder (80% or 1.98/100 person-years) were sudden or unexpected. Examining the underlying conditions, drug overdoses accounted for 45% or 1.1/100 person-years; bacterial sepsis, 25% or 0.6/100 person-years; liver disease, 26% or 0.6/100 person-years; and an undiagnosed AIDS condition, 9% or 0.2/100 person-years. Drug overdoses were the commonest cause of pre-AIDS death in this cohort of patients predominantly infected via IDU, but many of the sudden deaths had significant underlying pathology, which may have increased their susceptibility to an overdose of drugs. In future, death before an AIDS diagnosis should be classified into Medical or Expected Non-AIDS (MNA or ENA) and Sudden Non-AIDS (SNA).