Possible factors that might influence the outcome of continuous subcutaneous insulin infusion (CSII) treatment were investigated in forty-nine, type 1, diabetic patients with median treatment of 3.25 years in routine diabetes care. In nine patients pump treatment was discontinued largely within the first 6 months. Patients characterized as blue-collar workers discontinued pump treatment at a significantly higher rate. Of the remaining forty patients those with the longest treatment experience were of attracted interest. Among twenty patients who had used CSII three years or more improvement of metabolic control was most evident in patients with high haemoglobin A1c (HbA1c) values before CSII (R-square 0.673, P = 0.0001) and in subjects with low insulin requirements on injection therapy (R-square 0.334, P = 0.0076). This improvement was not explained by more frequent monitoring of blood glucose only. It was found that those who had the lowest body mass index (BMI) before pump treatment had the more pronounced increase of BMI, while those in the upper normal range had an unchanged or reduced body mass (R-square 0.271, P = 0.0115). We conclude that high HbA1c values, low insulin requirements and low body mass in conventional injection therapy are the most important factors when trying to achieve and maintain metabolic improvement in CSII treated type 1 diabetic patients.