OBJECTIVES: A new utility index derived of the SF-36, the SF-6D, was recently developed and has generated an increasing research in different groups of patients and has also been compared with other utility measures, as it is the EQ-5D. The purpose of present work was to evaluate the differences between the weighted and not weighted version of two utility measures: the Spanish SF-6D and EQ-5D. METHODS: A total of 1843 complete measures of the SF-36 and the EQ-5D (5 items and visual analogic scale-VAS) from 1283 patients who received a solid organ transplant (kidney, liver, heart or lung) were used. Data were collected at different moments during the first year after the surgery in the context of the Spanish Research Network on Transplantation. SF-6D values were calculated using the model proposed by its creator (weighted version) and without tariff values, as has been proposed by some authors (not weighted). EQ-5D values were calculated using Spanish VAS tariff (VAS-t), the time-trade off tariff (TTO-t) and also without tariff values (not weighted). Spearman correlation coefficients were calculated between SF-6D (weighted and not weighted) and EQ-5D values (VAS-t, TTO-t and not weighted). RESULTS: Mean value of weighted SF-6D was 0.67 (0.15) and not weighted, 0.72 (0.15). Mean values of EQ-5D VAS-t was 0.69 (0.24), of TTO-t, 0.70 (0.32) and of not weighted EQ-5D, 0.63 (0.33). SF-6D values had moderate correlation with EQ-5D VAS-t (r=0.734) and EQ-5D TTO-t (r=0.731) (both p<0.001). Using the SF-6D index without tariff values, it had a high correlation with the weighted version of SF-6D (r=0.969, p<0.001), and moderate with the EQ-5D VAS-t (r=0.754), EQ-5D TTO-t (r=0.750) and no weighted EQ-5D (r=0.784) (p<0.001). CONCLUSIONS: The efforts made to get the preferences values and calculate the weights in utility indexes do not seem to add enough information to make them worthy.