OBJECTIVES: Myotonic dystrophy type-1 (DM1) is a dominantly inherited disorder caused by an unstable CTG repeat expansion on chromosome 19.  Clinically the disease is associated with a wide variety of symptoms. The Myotonic Dystrophy Health Index (MDHI) is a disease-specific patient reported outcome measure designed to measure total health, and 17 of the most important areas of disease-specific health in DM1.  The items in this instrument were selected utilizing qualitative interviews and a cross-sectional study of 278 patients.  The MDHI’s total score (and each of the 17 subscales) are scored from 0-100 with 100 representing the highest level of disease.  The objective of this research is to evaluate the MDHI’s ability to differentiate between known groups of DM1 patients. METHODS: Each of the MDHI’s 17 subscales was completed by a group of DM1 patients.  DM1 respondents were divided into known groups by employment status, CTG repeat number (<300,300+), education (non-college graduate, college graduate), age (21-46,47+ years), and duration of symptoms (<21,21+ years).  The average subscale score and total MDHI score was calculated for each known group. RESULTS: On average, 138 DM1 patients completed each subscale.  The MDHI measured a higher disease burden in patients who were unemployed (MDHI total score: 41.5 vs. 25.0), less educated (41.4 vs. 33.8), had a longer duration of symptoms (41.3 vs. 34.6), were of older age (38.3 vs. 33.1) and those with a longer CTG repeat number (35.6 vs. 27.6). CONCLUSIONS: The MDHI and its subscales are capable of differentiating between known groups of DM1 patients who are suspected of having a higher burden of disease.