The difference between cirrus emissivities at 8 and 11 ¿m is sensitive to the mean effective ice crystal size of the cirrus cloud, De. By using single scattering properties of ice crystals shaped as planar polycrystals, diameters of up to about 70 ¿m can be retrieved, instead of up to 45 ¿m assuming spheres or hexagonal columns. The method described in this article is used for a global determination of mean effective ice crystal sizes of cirrus clouds from TOVS satellite observations. A sensitivity study of the De retrieval to uncertainties in hypotheses on ice crystal shape, size distributions, and temperature profiles, as well as in vertical and horizontal cloud heterogeneities shows that uncertainties can be as large as 30%. However, the TOVS data set is one of few data sets which provides global and long-term coverage. Having analyzed the years 1987--1991, it was found that measured effective ice crystal diameters De are stable from year to year. For 1990 a global median De of 53.5 ¿m was determined. Averages distinguishing ocean/land, season, and latitude lie between 23 ¿m in winter over Northern Hemisphere midlatitude land and 64 ¿m in the tropics. In general, larger Des are found in regions with higher atmospheric water vapor and for cirrus with a smaller effective emissivity.