A major study of the molecular gas surrounding young star clusters is underway. We are using the Columbia University 1.2–m millimeter-wave telescope to observe emission from the J=1→0 rotation transition of 12CO in the vicinities of 128 open star clusters. The survey region around each cluster is at least 10 cluster diameters in size, typically ≳ 5 square degrees. Sensitivity is sufficient to detect lines as weak as 1 K over a range in velocity ± 83 km/s centered on the cluster velocity and with a velocity resolution of 0.65 km/s. Clusters in this sample have well-determined distances ranging from 1 to 5 kpc, and ages ≳ 100 million years (Myr).