OBJECTIVE To evaluate the impact of syringe size on start-up delay and the time to reach 50% and 90% of target flow rates, using two commercially available syringe infusion pumps at infusion rates of < or =1 mL/hr. METHODS Two syringes (Terumo) of different size (10-mL and 50-mL), using two syringe infusion pumps (Pump A, Terumo Terufusion Infusion Pump TE-331; and Pump B, Braun Perfusor Compact S) were studied. Effective fluid delivery was measured at 0.4 mL/hr, 0.8 mL/hr, and 1.0 mL/hr for the initial 60 mins, using the gravimetric method. Instant flow was calculated as volume difference for every 1-min interval per minute. Start-up delay was defined as time in minutes of 0 flow from the start of infusion. Syringe placement, bubble removal, infusion line priming, and positioning were standardized for all measurements, using new syringes and infusion lines. Each experiment was repeated six times. Statistical analysis was performed, using a nonparametric test (Mann-Whitney U test). METHODS None. METHODS None. METHODS None. RESULTS Using the 50-mL syringe, the start-up delay was consistently higher and the time to reach 50% and 90% of target flow were significantly longer, independent of which syringe infusion pump was used. At every flow rate studied, the pumps did not reach the target flow rate before 60 mins with the 50-mL syringe. With the 10-mL syringe, target flow rate was achieved before 20 mins for both pumps. CONCLUSIONS Our findings demonstrate a clinically relevant impact of syringe size on syringe infusion pump performance at low flow rates. The time to reach 50% and 90% of target flow are significantly longer, using the 50-mL syringe compared with the 10-mL syringe, and the time to reach 50% of target flow is independent of the longer start-up delay. Based on our findings, we speculate that smaller syringe sizes and higher infusion rates are preferable for continuous drug infusions, particularly when prompt establishment of the drug effect is critical.