Fourteen consecutive patients with hepatocellular carcinoma (HCC) and diaphragmatic invasion found during operation and confirmed histopathologically were studied. They were treated by resection of the liver tumour en bloc with part of the diaphragm. Eight of the 14 patients had a diaphragmatic hump on chest radiography. Preoperative ultrasonography, computed tomography and hepatic angiography were unable to diagnose diaphragmatic invasion in these patients. A control group of 14 closely matched patients were selected from 98 undergoing curative resection for HCC during the period of the study. No evidence of a diaphragmatic hump was found in the control patients (P < 0.001). There was no significant difference in survival, operative morbidity or mortality rates between the two groups. In patients with HCC situated in the dome of the liver, the presence of a diaphragmatic hump on chest radiography strongly suggests invasion of the diaphragm. There was no difference in outcome in patients with or without diaphragmatic invasion; the tumour should be resected if the patient is fit for operation.