OBJECTIVES: To document the psychometric properties and estimate a minimal important difference (MID) for the Activity Impairment Assessment (AIA) questionnaire in patients with acute bacterial sinusitis. The AIA has previously been validated in people with uncomplicated urinary tract infections.  METHODS: Secondary psychometric analysis was undertaken on data from a phase III trial to evaluate the efficacy and safety of moxifloxacin over 5 consecutive days in the treatment of acute bacterial sinusitis (N=374). Trial patients also completed the RAND SF-36, the Sinonasal Outcomes Test-16 (SNOT-16) and a global rating of change.  The AIA has 5 items and is scored as a uni-dimensional measure of the impact of disease on an individual’s work or regular activities.  Internal consistency and construct validity (against SF-36 and SNOT-16) were assessed at baseline.  Sensitivity and MID were assessed by comparing test of cure data against baseline using statistical methods (effect size (ES), standardised response mean (SRM); and standard error of measurement (SEM) and ½ standard deviation (½SD)) and anchor methods against the smallest change on the global rating of change.  Missing data were imputed using last observation carried forward.  RESULTS:  Internal consistency (α=0.940) and construct validity (SF-36 Role Function r=-0.66; Vitality r=0.62, Social Function r=0.71, Pain r=0.64, SNOT-16 r=0.67 all P<0.001) were confirmed with strong correlations.  The AIA was sensitive (ES =0.90; SRM=0.82).  The MID estimated by the anchor method was 2.63; but MID was lower by distributional estimates (SEM=1.39, ½SD = 2.84).  CONCLUSIONS:The study confirms the good psychometric properties of the AIA in patients with acute bacterial sinusitis and demonstrates the sensitivity and MID for this measure.  Distributional MID estimates confirmed the anchor based approach.