In Hebden v. Workmen's Comp. Appeal Bd. (Bethenergy Mines), Inc.), 534 Pa. 327, 632 A.2d 1302 (1993), the claimant was awarded partial disability benefits for occupationally acquired pulmonary lung disease (coal workers' pneumoconiosis), a progressive disease. Thereafter, the employer sought to terminate benefits, contending that the claimant was no longer disabled from the pulmonary disease. The employer offered the testimony of two experts. One expert opined that the claimant did not currently have pneumoconiosis, which, he admitted on cross-examination, also indicated that the claimant did not have the condition when benefits were originally awarded. The other expert also opined that the claimant did not have pneumoconiosis, but rather suffered from a non-occupational respiratory ailment. The WCJ credited employer's witnesses, finding that the claimant's disability ceased to exist and that res judicata did not bar the termination of benefits. The order was affirmed on appeal. The Supreme Court reversed, opining that res judicata prevents the employer from relitigating in a termination petition the original medical diagnosis underlying the original award of benefits. Noting that the United States Supreme Court had determined in Usery v. Turner Elkhorn Mining Co., 428 U.S. 1, 49 L. Ed. 2d 752, 96 S. Ct. 2882 (1976), that coal workers' pneumoconiosis is irreversible, our Supreme Court found that the employer's medical evidence failed to rebut the presumption that pneumoconiosis is irreversible.