Seventeen insulin dependent diabetics were studied after two to four weeks of insulin treatment in a situation approximating to their normal daily life. Some endogenous insulin secretion, assessed by plasma C-peptide determinations, was present in all. Plasma C-peptide concentration was positively correlated with the blood glucose concentration and increased after breakfast, lunch and dinner (p less than 0.01); both peak values and relative increases were lower than those observed in normal subjects (p less than 0.01). The highest insulin secretory capacity was found in subjects with the least unstable blood glucose concentration (r=0.57, p less than 0.03), and these patients required the smallest insulin doses (r=0.54, P less than 0.04). These findings demonstrate the metabolic importance of a preserved B-cell function.