We have studied the impact of home blood glucose measurements with the Ames Dextrostix/Eyetone system on the metabolic control of 17 insulin-dependent, ketosis-prone, unstable diabetic patients. Thirteen patients (69 patient-months) were enrolled in a comprehensive program designed to achieve excellent metabolic control and which included the daily use of the Eyetone meter. Eleven of these patients were diabetic kidney recipients on daily prednisone. The improvement of the metabolic control, as assessed by daily home blood glucose measurements, was highly significant (P < 0.001) in 11 patients (85%). The control was still inadequate, however, in at least seven patients. In four unstable diabetic patients who were given the meter for 4 mo but were not followed closely during this period, modest or no improvement in metabolic control was observed. The Eyetone meter proved highly precise and accurate in our patients' hands. Several patients achieved a coefficient of variation of less than 5 (in 10 consecutive determinations) and a correlation coefficient with the Beckman glucose analyzer of 0.96. We concluded that the metabolic control can be markedly improved in the most unstable diabetic patients with home blood glucose measurements, provided the patients receive adequate and frequent encouragement and diabetic teaching. Without the latter, at least in unstable diabetes, whether patients are compliant or not, the instrument may have little impact on control.