The purpose of this study is to present data comparing the penetration of cryolesions created by various sizes and shapes of cryoprobes in human cadaveric myocardium, fat, and tissue of the central fibrous body. Ten cryolesions were made for each combination of tissue and cryoprobe studied. All cryolesions enlarged most rapidly during the first minute of cryothermia (p < 0.01). Maximal cryothermic penetration into nontrabeculated myocardium was 8.5 ± 0.5 mm (15-mm flat probe) and 6.1 ± 1.0 mm (5-mm small probe). Maximal cryothermic penetration into trabeculated myocardium was 9.4 ± 1.0 mm (10-mm cone-tipped probe) and 7.4 ± 0.5 mm (10-mm flat probe). Maximal cryothermic penetration into fat was 4.7 ± 0.7 mm (15-mm flat probe) and 3.9 ± 0.7 mm (5-mm flat probe). The deeper penetration of cryothermia into myocardium as compared with fat (p < 0.05) is related to the lower thermal conductivity of fat. Maximal cryothermic penetration of the central fibrous body was similar to that of the myocardium with transmural freezing of the central fibrous body after 4.4 ± 0.3 minutes of cryothermia. These data can be used when determining the optimal cryothermic exposure for ablation of arrhythmogenic tissue. © 1992.