The "Last Millennium" simulation (LM, years 850-1850 ) is part of CCSM4 contribution to CMIP5. It allows an evaluation of the capability of the model to capture observed variability on multidecadal and longer time scales. It also provides a means of assessing what portion of the variability is attributable to external forcing or internal variability. Our study shows the CCSM4 LM simulation reproduces many large-scale climate patterns suggested by historical and proxy-data records. Its response to large volcanic eruptions captures some reconstructed patterns of temperature changes over Europe and North America, but not those of precipitation in the Asian monsoon region. Modes of circulation variability are also analyzed and compared with some proxy reconstructions.