Muscle capillary basement membrane width (MCBMW) was measured in 18 myotonic dystrophy patients and compared with that in age- and sex-matched normal and diabetic subjects. The MCBMW in myotonic dystrophy patients (773 ± 258 Å) was significantly thinner than normal subjects (925 ± 181 Å, P<0.05) or in diabetics (1224 ± 614 Å, P<0.01). An increase in MCBMW with advancing age was present in all groups but was greatest in the myotonic dystrophy groups (r=+0.59, P<0.01). There was no relation between MCBMW and either the degree of glucose intolerance or insulin hypersecretion in the myotonic dystrophy group, though none had fasting hyperglycemia. This is the first report of a condition associated with thinner-than-normal capillary basement membrane.