Blood glucose concentrations during normal daily activities were measured in 106 patients with maturity-onset diabetes from capillary blood samples collected on to filter paper. Samples were taken before and two hours after main meals, before going to bed, and, in 51 cases, during the night. Fasting and mid-morning values were closely correlated with the mean values over 24 hours irrespective of the type of anti-diabetic treatment being given. Postprandial blood glucose concentrations remained below 11.5 mmol/l (207 mg/100 ml) when the fasting blood glucose value was 7.0 mmol/l (126 mg/100 ml) or less, and repeated fasting blood glucose values exceeding 7.0 mmol/l were associated with raised blood glycosylated haemoglobin concentrations. Diabetic control in maturity-onset diabetes may be satisfactorily monitored by regular measurement of fasting or mid-morning blood glucose values.