The aim of this study was to analyze the absence of adjustment of insulin doses in type 1 diabetic patients with poorly controlled diabetes. Twenty-eight patients (HbA1c higher than 8.5% during the last 6 months, performing at least three capillary blood glucose determinations per day), completed a questionnaire on the degree of confidence in their own knowledge, the nature of their health beliefs, their fear of hypoglycemia, their own appreciation on how they adjust their insulin doses (subjective score). An analysis of their diabetes logbook provided an objective score of the adjustment of doses actually performed. The results show that the subjective and objective scores of adjustment were not significantly correlated. Further there was a significant negative correlation between the score of uncertainty on knowledge and the subjective score of adjustment of the insulin doses, but not with the objective score. There was a significant correlationbetween the score of positive health beliefs and the subjective score of adjustment of the insulin doses, but not with the objective score. No correlation was found between the score of fear of hypoglycemia and the subjective score of adjustment of the insulin doses. Correlation with the objective score was higher, but not significant. Actually, the fear of hypoglycemia was the most frequently given reason for not adjusting the insulin doses, when the question was asked to the patients with an open answer. This study illustrates the difference between thinking and doing. It also shows that the degree of confidence in one’s own knowledge, the health beliefs, and the fear of hypoglycemia differently influence the perception that the patients have of their behavior, and what they really do. © 2004 Elsevier Ireland Ltd. All rights reserved.