Twenty-four per cent of 2315 patients admitted to the intensive care unit of St Thomas's Hospital in the 4 year period from November 1969 became colonized or infected with Klebsiella species. Capsular typing of 986 klebsiella isolates from 551 patients showed that there were 695 patient-isolates, mostly derived from the respiratory tract. Capsular types 47 and 21 were the commonest types and together accounted for 19.9% of the patient-isolates. The 14 commonest types accounted for 47.3% of all patient-isolates and all these strains showed clustering, strongly suggesting a changing common source, cross infection, or both.