Hemoglobin A1 (HbA1) blood levels were examined in 34 cystic fibrosis (CF) patients aged 1 1/2 to 20 1/2 yr. The mean (+/- SD) HbA1 concentration in the 34 patients was 7.97 +/- 1.16--significantly higher than that in 150 normal children (6.8 +/- 0.8). Oral glucose tolerance tests (OGTT) were performed in 11 patients and blood glucose and insulin concentrations were measured. There was no difference in the fasting blood glucose values between the two groups. Eight children had a normal OGTT, and three showed an impaired OGTT with a delayed peak response in insulin levels. There was a decreased insulin response in all the CF patients. No correlation was found between HbA1 concentration and the age of the patients, nor between HbA1 and the clinical status (Shwachman score). We conclude that insulinopenia in children with CF is not always accompanied by an increment in HbA1, although serial determinations of its level may reveal deterioration in glucose tolerance.