We studied the relation between echographic tumor elevation (measured by standardized A-scan echography) and histologic tumor thickness (measured from histologic slides with an ocular micrometer) in a series of 53 choroidal and ciliary body melanomas enucleated within one month of the most recent echographic measurement. Histologic height was less than echographic height for all sizes and locations studied, with a correlation coefficient of r = .926. The following equations for straight lines described the relationship: Echographic height = 1.964 + (1.042 X histologic height). Histologic height = -0.660 + (0.823 X echographic height). Thus, current thickness criteria for small (less than 3 mm), medium (3 to 5 mm), and large (more than 5 mm) melanomas, based on histologic measurements, may be translated to the following echographic terms: small, less than 5.2 mm; medium, 5.2 to 7.2 mm; and large, more than 7.2 mm. The source of this difference is a variable degree of tumor shrinkage induced by fixation and histologic preparation.