BACKGROUND We investigated whether the arrow on a continuous glucose monitoring system (CGMS) screen predicts the course of the capillary glucose level 15 min later. METHODS Twenty-three patients with type 1 diabetes (age, 40±13 years; diabetes duration, 19±12 years; hemoglobin A1c, 8.5±1.5%) admitted for education in the use of a CGMS performed 242 observations: the arrow was noted at time 0, and the interstitial and capillary glucose levels were noted at time 0 and 15 min later. RESULTS The capillary glucose courses were -15±28 mg/dL after a descending arrow (n=55), +1±23 mg/dL after a stable arrow (n=147) (P<0.001 vs. descending), and +2±23 mg/dL after an ascending arrow (n=40) (P<0.01 vs. descending), with similar findings for the 67 observations after an interstitial glucose level <100 mg/dL. There were 4.5% grossly erroneous arrows: six descending with later increasing and five ascending with later decreasing capillary glucose. CONCLUSIONS Although there is a large room for improvement, the arrow on the CGMS screen does predict the decline in capillary glucose 15 min later.