Carbon monoxide abundances in the middle atmosphere are retrieved from infrared measurements of the emission of this molecule at 4.6 μm taken by the improved stratospheric and mesospheric sounder (ISAMS) on board the Upper Atmosphere Research Satellite. In addition to a limited signal-to-noise ratio, the measured radiances include significant contamination by other compounds in the stratosphere and show clear non-local thermodynamic equilibrium effects in the mesosphere which also affect the stratospheric limb views. The application of the ISAMS retrieval method to these measurements and the particular method followed to deal with these difficulties are described. The actions carried out to understand and validate the obtained CO abundances are detailed. They include analysis of the radiances, sensitivity studies of the product to retrieval parameters, evaluation of the main sources of systematic and random errors, and comparisons with theoretical model predictions and with the few correlative measurements available. The gross features observed in the CO distribution present good agreement with dynamical-chemical models of the middle atmosphere. The aspects of the retrieval that need further improvement are identified, and the global quality of the database and the expectations for its scientific use are discussed. ¿ American Geophysical Union 1996