SUMMARYOver the period 1995–2012, the incidence of invasive meningococcal disease (IMD) caused byNeisseria meningitidisserogroup Y (NmY) increased significantly in Sweden. This is mainly due to the emergence of a predominant cluster named strain type YI subtype 1, belonging to the ST-23 clonal complex (cc). The aim of this study was to examine the clinical picture of patients with invasive disease caused by NmY and to analyse whether the predominant cluster exhibits certain clinical characteristics that might explain the increased incidence. In this retrospective observational study, the medical records available from patients with IMD caused by Nm serogroup Y in Sweden between 1995 and 2012 were systematically reviewed. Patient characteristics, in-hospital findings and outcome were studied and differences between the dominating cluster and other isolates were analysed. Medical records from 175 of 191 patients were retrieved. The median age was 62 years. The all-cause mortality within 30 days of admission was 9% (15/175) in the whole material; 4% (2/54) in the cohort with strain type YI subtype 1 and 11% (12/121) among patients with other isolates. Thirty-three per cent of the patients were diagnosed with meningitis, 19% with pneumonia, 10% with arthritis and 35% were found to have bacteraemia but no apparent organ manifestation. This survey included cases with an aggressive clinical course as well as cases with a relatively mild clinical presentation. There was a trend towards lower mortality and less-severe disease in the cohort with strain type YI subtype 1 compared with the group with other isolates.