Using the data gathered from the Advanced Very High Resolution Radiometer (AVHRR) 0.63 and 3.7µm channels, an algorithm for the inference of cirrus cloud optical depth and mean effective size has been developed for the first time. This scheme is based on the correlation between the 3.7µm (total) and 0.63µm radiances that are constructed from radiative transfer calculations involving ice crystal clouds. Application of the algorithm to AVHRR channels has been performed by using data sets that were collected during the First ISCCP Regional Experiment, Phase II, Cirrus Intensive Field Observation (FIRE-II-IFO; November-December 1991) at Coffeyville, Kansas. For validation, the in-situ data collected by the balloon-borne replicator and airborne 2-D probes that were collocated with AVHRR pixels were carefully analyzed for five cases involving single and multiple cirrus cloud layers. We demonstrate that the retrieved cirrus cloud optical depths and mean effective sizes compare reasonably well with those determined from the in situ analyses.