OBJECTIVES: The International Subarachnoid Aneurysm Trial (ISAT) has revealed significantly better clinical outcomes for patients randomised to endovascular treatment as measured using the modified Rankin scale (mRS). ISAT compared the proportion of patients with a mRS grade of three or over (indicates higher level of impairment and dependency) following endovascular or neurosurgical treatment. The HRQL data were explored to determine whether there are differences in patients ranked 0-2 on the mRS as well as three or over. METHODS: A sub-sample of ISAT patients from eight UK centres completed a thorough assessment of HRQL (SF-36 and Functional Limitations Profile) and cognitive function at 12 months following treatment. HRQL data are reported here. Differences were tested using one-way ANOVA with post-hoc comparisons (LSD) between mRS grades. RESULTS: Every domain of the SF-36 declines significantly with deteriorating mRS grade. The post-hoc comparisons reveal that each grade is significantly worse than the next lower grade, apart from grades four and five. Patients within the 0-2 mRS range still report substantial declines in HRQL (PF is 90.1 at grade zero (n=137), to 66.6 at grade 2 (n=144); VT is 71.6 at grade zero and 39.9 at grade 2; BP is 93.7 at grade zero and 64.0 at grade two). Sample sizes for grades three and over are small, but the data indicate that little sensitivity to differences in HRQL outcomes in patients at these higher mRS grades. CONCLUSIONS: The HRQL data greatly elucidates the differences in health status for patients at different mRS grades. The HRQL data demonstrate that simply categorising patients using a single cut-off score on the mRS is a very crude way of measuring outcomes. Most of the decline in HRQL scores occurs in the 0-2 range.