The Cryogenic Telescope Assembly (CTA) on the Spitzer Space Telescope employs a revolutionary warm launch design. Unlike previous space cryogenic telescopes, the Spitzer telescope is mounted outside of the cryostat and was launched at ambient temperature. The telescope was cooled through a combination of passive radiation and controlled vapor cooling from the superfluid helium cryostat. Launched in August 2003 with 49 kg of helium, the 0.85-meter telescope cooled to below 5.5 K within the initial 45 days of flight in accordance with analytical predictions. Despite an aggressive schedule of instrument initialization and checkout testing during the first two months of flight, the CTA met the temperature requirements for all checkout activities. The transient flight performance of this multi-stage thermal/cryogenic system has been found to agree well with pre-launch predictions over the broad temperature range. With an emphasis on early flight cool-down behavior, this report highlights the pre-launch cryostat preparation, the thermal behavior during cryostat blow-down, comparisons to pre- and post-launch model predictions, and in-flight helium mass measurement. The post cool down performance and rate of helium use is also discussed.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.