We examined the clinical features of 14 brittle diabetic patients, all of whom had failed to achieve near-normoglycemia during the ordinarily optimal treatment regimen of continuous subcutaneous insulin infusion. All patients were female. Seven patients had obvious family problems such as divorce or parental conflict. Type A (12 patients) suffered mainly from frequent, unpredictable episodes of ketoacidosis, necessitating many emergency admissions to a hospital; type B (two patients) complained of frequent, unpredictable, symptomatic hypoglycemia, short of coma and not requiring admission to a hospital. Type A brittle diabetic individuals were compared with 12 stable diabetic patients matched for age, sex, and duration of diabetes. The type A diabetic patients were young (mean age: 19.2 yr; range: 13–27 yr) with a wide range of duration of diabetes (2.5–23 yr) and brittleness (1–11 yr). Although their body mass index (BMI) was not significantly different from the stable diabetic subjects, many type A brittle patients were overweight, despite usually constant ketosis and hyperglycemia. The daily subcutaneous insulin dose was significantly higher than for stable diabetic individuals (mean ± SEM: 7.1 ± 2.6 versus 0.96 ± 0.12 U/kg), reflecting, perhaps, a subcutaneous absorption barrier in brittle patients. There was a significant correlation between age of onset of brittleness and age at menarche, with brittleness starting in most patients 2–5 yr after a normal-age menarche. This may implicate the hormonal and/or psychological changes at this time. Most type A brittle patients had secondary amenorrhea or oligomenorrhea at the time of study. Type B patients were older (32 and 38 yr) with long duration of diabetes (24 and 28 yr) and brittleness (20 and 28 yr). They were receiving a normal subcutaneous insulin dose (0.5 and 0.8 U/kg) and had a regular menses. Furthermore, brittleness appeared before menarche in these patients.