The Clinitest was used to assess the effect of inpatient treatment in 24 labile insulin-dependent diabetics with a portable open loop insulin infusion device. The Clinitest results one month prior to hospitalization were compared with those during 3 months following inpatient treatment. During the first month after the hospital stay 21 patients (88%) were better controlled. Only 15 (63%) maintained better control for the whole period of 3 months. After the hospital stay the patients experienced an average of 4 more slight hypoglycemic episodes than before hospitalization. In patients who were better controlled for the whole period of 3 months the insulin dose had been adjusted considerably. Besides inadequate daily adjustment of the insulin dose, half the patients were not sufficiently motivated and did not cooperate with their diabetologists. One third of the patients did not follow a diet and one fifth practiced inadequate insulin injection techniques. These were the major causes of poor control prior to hospitalization. The results show that short term treatment with an open loop insulin infusion device is worth while for all diabetics in whom less than one third of all Clinitests are sugarfree, and also for all those in whom there are doubts about the actual insulin requirement.