Strains of Staphylococcus aureus resistant to gentamicin and methicillin first appeared in Dublin hospitals in 1976, and rapidly became widely disseminated. The number of patients infected or colonised increased throughout the period of study, especially in 1979 and 1980. Most isolates were from burns, surgical wounds and traumatic skin lesions. During the 12 months after first isolation of these multiply antibiotic resistant strains, colonisation or minor infection was the usual event. Invasive infection such as bacteraemia, deep wound sepsis and osteomyelitis was rarely seen. Subsequently, as the number of patients from whom these organisms were isolated increased, bacteraemia and other severe infection became more common. The predominant phage type of S. aureus changed with the progression of the outbreak. Isolates of different phage type were sometimes found in a single lesion, or in different sites in one patient. By the second half of 1980, most isolates were untypable or typed only with an experimental phage.