When assembling lithium-ion cells into functional battery packs, it is common to connect multiple cells in parallel. Here we present experimental and modeling results demonstrating that, when lithium ion cells are connected in parallel and cycled at high rate, matching of internal resistance is important in ensuring long cycle life of the battery pack. Specifically, a 20% difference in cell internal resistance between two cells cycled in parallel can lead to approximately 40% reduction in cycle life when compared to two cells parallel-connected with very similar internal resistance. We show that an internal resistance mismatch leads to high current into each cell during part of the charging cycle. Since capacity fading is strongly dependent on temperature, and hence on charging rate when this rate is sufficiently high, the high current leads to substantially accelerated capacity fade in both cells. A model, based on the formation of a solid-electrolyte interphase, is able to explain the dependence of lifetime on resistance mismatch, and also identifies the importance of random sudden capacity losses.