OBJECTIVES: Little is known about the health-related quality of life (HRQL) in patients with primary antibody deficiencies (PAD) and their need of lifelong immunoglobulin G (IgG) replacement therapy. Daly et al. (1991) developed a questionnaire, the Life Quality Index (LQI), consisting of one sale of 15 items related to IgG treatment. The higher the score, the better the treatment- specific HRQL. The construct validity has never been shown, nor has the instrument been related to patient-reported data. METHODS: Of 58 patients (17 children, 41 adults) with PAD were treated with weekly IgG infusions (Beriglobin P, Aventis Behring, Marburg) for 10 months. Beside other instruments, the patients answered the LQI and a single-item related to their change in health status at baseline, 6- and 10-months. Construct validity was evaluated using explorative factor analysis using the Kaiser criterion and a varimax rotation. For the evaluation of clinical validity an anchor-based approach was applied, i.e. the patients' self-assessments of their health-status changes over time were related to LQI changes over time. RESULTS: There was a noteworthy improvement for 9 LQI items in adults and for 12 LQI items in children from baseline to 10-months. Exploratory factor analysis suggested a three-factor solution that accounted for 52% of the total variance. For patients reporting an improvement in their health status, all mean changes in LQI scores were positive, and in 10 of 15 items changes larger than 0.5 were observed. Thus, an improved self-perceived health led to improved LQI scores. CONCLUSION: The LQI is well suited to capture treatment-specific aspects of HRQL in patients with PAD receiving IgG. The items could be aggregated into three sub-scales. The anchor-based approach indicates that the LQI may serve as a valid instrument to assess the patients' perceptions of their treatment.