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A long-term decrease in mean cloud optical depth and mean cloud top temperature exists in the International Satellite Cloud Climatology Project (ISCCP) dataset for the July 1983 to December 1990 time period equatorward of 50° latitude. The decrease in optical depth of 2 is large compared to its mean of 4.6 for this period. The nature of this decrease suggests that it is not real, but rather an artifact of incomplete normalization of the visible channel on successive polar orbiters used as the calibration standard for ISCCP. This decrease has large impacts on the amounts of different cloud types defined by ISCCP, especially for cloud types whose mean amount is small. The decrease in mean cloud top temperature of 5 K has a smaller effect on the amounts of various cloud types. More accurate calibration techniques are needed in order to establish long-term climate trends from ISCCP.