We performed a cross-cultural adaptation of the McGill Pain Questionnaire (MPQ) from English to Spanish for studying Mexican Americans in South Texas. Each of the 78 single-word pain descriptors in the original MPQ was translated into Spanish by a panel of nine bilingual health researchers, preserving the original structure of the questionnaire. The pain-intensity content (PIC) of the words in each language was then rated on a 100 mm visual analog scale by 8 bilingual health care providers and 10 bilingual health-care consumers. The correlation between Spanish and English average PIC ratings was strong (r = 0.85 for providers, r = 0.80 for consumers). The translated Spanish version was compared to the original English in a group of 50 bilingual Mexican-American patients with musculoskeletal pain, who completed the MPQ in both languages. There was no difference in Average Pain Rating Index between the Spanish and English versions (29.8 +/- 14.7 vs 29.1 +/- 15.8, p = 0.55), and agreement between the two language versions was almost perfect (ri = 0.85). Test-retest reliability was measured in two groups of hospitalized patients (25 per group), one composed of monolingual Spanish speakers and the other of monolingual English speakers. Each subject completed the MPQ, the McGill Pain Map, two 10-cm visual analog scales measuring pain now and within the past week, the bodily pain items of the MOS-SF36 survey, and the Modified Health Assessment Questionnaire, on two occasions one day apart. Test-retest reliability of the Spanish and English components of the MPQ was not significantly different and was comparable to that of the other pain and health status instruments. We conclude that the Spanish MPQ is cross-culturally equivalent to the original English and has similar concurrent validity and reliability. This questionnaire is suitable for cross-cultural studies of pain comparing Spanish-speaking Mexican Americans with English-speaking members of the same and other ethnic groups.