Hi, Please consider the following : > x = as.integer(2^30-1) [1] 1073741823 > sum(c(rep(x, 10000000), rep(-x,9999999))) [1] 1073741824 Tested on 2.15.2 and a recent R-devel (r62132). I'm wondering if s in isum could be LDOUBLE instead of double, like rsum, to fix this edge case? https://svn.r-project.org/R/trunk/src/main/summary.c Thanks, Matthew