The present study was undertaken to evaluate the CIE overcast sky model for Japan using the data now emerging from the CIE International Daylight Measurement Programme. Kyushu University in Japan is the national and international co-ordinating centre for this programme. Using data from only those days which experienced a complete overcast it has been shown herein that the above model always underestimates the average vertical illuminance. The analysis indicates that a uniform sky is a better representation of the overcast luminance distribution.