BACKGROUND: The prevalence of comorbidities increases with age but preference-based utilities are generally obtained from cohorts who have a single condition. This can cause problems when populating health states in economic models which represent more than one condition. Analysts use the mean utilities from the cohorts with the single conditions to estimate the mean utility for a cohort with comorbidities. There is currently no consensus on which is the most appropriate method to combine these data and the different techniques can produce very different results. OBJECTIVE: To compare the accuracy of different methods (minimum, multiplicative, additive, a linear regression) used to estimate health state utility values for comorbid health conditions.