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 the present work is to validate this index in the Spanish version of SF-36 with respect to the Spanish version of the EQ-5D. METHODS: A total of 1843 complete measures of the SF-36 (version 2) 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. EQ-5D values were calculated using Spanish VAS tariff (VAS-t) and time-trade off tariff (TTO-t). Spearman correlation coefficients were calculated between SF-6D and EQ-5D values. RESULTS: Mean value (SD) of SF-6D was 0.67 (0.15) (range 0.3-1.0), of EQ-5D VAS-t, 0.69 (0.24) (range -0.08–1.0) and of TTO-t, 0.70 (0.32) (range -0.7–1). Percentage of subjects scoring the maximum was 1.1% for SF-6D, and 24.5% for the EQ-5D. 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). CONCLUSIONS: The SF-6D index derived from the Spanish version of SF-36 seem to be a valid utility index to be used with the SF-36 databases from studies made in Spain. However the moderate correlation between both utility measures indicates that probably they partially measure different concepts.