Objective. The Stanford Health Assessment Questionnaire Disability Index (HAQ) is the gold standard functional status questionnaire in rheumatology, but it is lengthy. Three shorter versions, the modified HAQ (MHAQ), the Multidimensional HAQ (MDHAQ), and the HAQII are often used in outcomes research as HAQ substitutes. We developed conversion formulas between these modified versions and the original HAQ. Methods. Analysis was limited to the comparison of rheumatoid arthritis (RA) patients at a random observation when the HAQ was recorded in conjunction with the MHAQ (n = 29,596), the MDHAQ (n = 13,665), or the HAQII (n = 15,823). Development models were randomly limited to 80% of the data (development sample) and the remaining 20% was used for model validation. Results. Two conversion formulas were developed for each of the MHAQ, the MDHAQ, and the HAQII: a short model and a long model inclusive of questions common to both the modified measures and the original HAQ. Short models explained 81-83%, and long models 82-86%, of the variance. Predicted HAQ values of zero were assigned to all cases with an MDHAQ or HAQII score of zero, with remaining cases used for model estimation. Bland-Altman plots demonstrated good concordance between actual and predicted values for each measure. The validation sample closely approximated the results from the development sample (0.005 ≤ ΔR 2 ≤ 0.009) for each measure. Conclusion. We have developed and validated highly accurate conversion formulas from the MHAQ, MDHAQ, and HAQII to the original HAQ in a large sample of RA patients. The developed models are useful for conversion of measures in the research setting. Because of substantial variability at the individual patient level, application of the formulas to individual patients is inadvisable. © 2010, American College of Rheumatology.