OBJECTIVES: The Dermatology Life Quality Index (DLQI) is a widely-used HRQL measure. The instrument is intended for use by patients with any skin disease. The aim of this study was to assess the scaling properties of the DLQI and whether it is free from differential item functioning (DIF). METHODS: DLQI data collected in atopic dermatitis (AD) and psoriasis studies in the UK were subjected to Rasch (one-parameter logistic item response theory) analysis. Fit to the Rasch model was examined via Chi2 statistics and assessments of DIF related to gender, age and type of skin disease were made. RESULTS: Sample: Psoriasis study: n=148 (49.7% male; mean age 45.1 +/-14.9; mean illness duration 20.9 +/- 13.5; mean DLQI score 8.7+/-6.7); AD study: n=286 (29.4% male; mean age 44.9 +/-16.4; mean illness duration 29.0 +/-16.7; mean DLQI score 7.0 (+/-5.1). The DLQI showed significant misfit to the Rasch model in psoriasis and AD - indicating that the instrument is not unidimensional. Several DLQI items exhibited DIF by age and gender. Four of the ten items in the measure exhibited DIF by disease. CONCLUSIONS: The DLQI was found to misfit the Rasch model in both patient samples, indicating that it is unsafe to use the total score on the measure. Its validity is further compromised by DIF associated with age and gender which indicates that items work differently with different subgroups of patients. For example, the item ‘How much has your skin influenced the clothes you wear?’ exhibited DIF by gender in both psoriasis and AD samples, indicating that women were significantly more likely than men (at the same level of QoL) to affirm the item. Finally, the extensive DIF by disease observed indicates that it is not valid to compare the scores of AD and psoriasis patients using this measure.