In April–May 2000, eight boreholes were drilled to ∼15–20 m depth on the Devon Ice Cap. ¹³⁷Cs γ activity profiles of each borehole showed a peak count rate at depth that is associated with fallout from atmospheric thermonuclear weapons testing in 1963. Snow, firn, and ice densities were measured at each core site and were used to estimate the average pattern of mass balance across the accumulation zone of the ice cap over the period 1963–2000. The average mass balance across the entire ice cap for the period 1960–2000 was also estimated using a degree-day model driven by data derived from on-ice temperature sensors and long-term measurements at Resolute Bay. Best fitting degree-day factors were determined for different sectors of the ice cap by comparing model output with repeated annual mass balance measurements made along two transects (Koerner, 1970). The results suggest that the ice cap has lost ∼1.6 km³ water per year, equivalent to a mean net mass balance of approximately −0.13 m We a⁻¹. Estimates of the mean mass balance for individual drainage basins reveal regions of positive and negative mass balance that are consistent with remotely sensed observations of advancing and retreating ice cap margins, respectively.