Use of continuous subcutaneous insulin infusion (CSII) therapy has increased among patients with type 1 diabetes. This study was performed: (1) to evaluate the effect of CSII on diabetes control in children and young adults, (2) to detect effects of CSII on weight, body mass index (BMI), and insulin requirements, (3) to investigate seasonal variation in diabetes control during CSII therapy, and (4) to investigate the effect of season of initiation of CSII on glycemic control. Thirty-nine patients, ranging in age from 10.1 to 20.5 years, with type 1 diabetes were studied. Quarterly data over 12 months preceding and following CSII initiation were obtained retrospectively. Variables were compared over similar time periods. SAS was used for descriptive and paired t test analysis. (1) Mean blood glucose level was significantly lower at 3 months but not different from baseline at 6, 9, and 12 months post-CSII. (2) Glycosylated hemoglobin (HbA1c) was significantly lower at 3 and 6 months but not at 9 and 12 months post-CSII. (3) There was no significant difference in the frequency of hypoglycemia or hyperglycemia at any of the time periods studied. (4) There was an initial but unsustained decrease in daily weight-adjusted insulin requirements after CSII. (5) There was a rapid, sustained increase in weight and BMI following CSII in females. (6) Frequency of ketoacidosis decreased in two patients. (7) There was no seasonal variation in weight change, HbA1c, or frequency of measured hypoglycemic episodes with CSII. (8) There was some effect of the season of initiation of CSII therapy on glycemic control. Thus, (1) CSII glycemic benefits may not be sustained, (2) weight gain is a significant effect of CSII in adolescent females, and (3) CSII may be a means of decreasing ketoacidosis episodes, and eliminating seasonal variability in diabetes control.