Objective: This study was done to identify factors influencing long-term metabolic control in youth with type 1 diabetes mellitus (T1DM) treated with an insulin pump. Research Design and Methods: Data were obtained from retrospective chart review of 113 patients (52 males) with T1DM treated with an insulin pump for up to 7 years. Their mean±SD age at diagnosis of T1DM was 9.7±5.1 years, and that at pump therapy initiation was 13.8±6.1 years. Linear trends and changes in hemoglobin A1c (HbA1c) levels following pump insertion were evaluated according to gender, metabolic control prior to initiation of pump therapy, time from diagnosis of diabetes until pump therapy, age at initiation, and the duration of pump treatment. Results: Mean HbA1c levels of patients with good baseline metabolic control (HbA1c level ≤7.5%) were significantly lower during the whole follow-up period (7.2±0.9%) compared to the moderate (baseline HbA1c level 7.5% to ≤9%) and poor (baseline HbA1c level >9%) control groups (8.1±0.9% and 8.2±1.1%, respectively; P<0.001). However, with time a significant trend for increasing HbA1c level was demonstrated in the group with good metabolic control (P value for trend=0.004). HbA1c levels of patients with poor baseline metabolic control decreased significantly immediately after pump initiation (9.4±1.6% vs. 8.0±1.2%, P=0.0001) and thereafter remained stable (P value for trend=0.54). In the multivariable analyses, baseline HbA1c level ≤7.5%, duration of ≤1 year between diagnosis of diabetes and pump initiation, and younger age at pump initiation were independently associated with lower HbA1c levels during long-term follow-up. Conclusions: Long-term response to pump treatment was dependent on baseline metabolic control. Early pump treatment had a significant long-term impact on metabolic control.