Portal haemodynamic studies were carried out in 11 subjects, seven with chronic myeloid leukaemia and four with chronic lymphatic leukaemia, and results compared with those obtained in five patients with 'idiopathic' splenomegaly and with control subjects. All 11 patients with chronic leukaemia had intrasplenic pressures above 11 mmHg and of these four had pressures above 20 mmHg. Portosystemic collaterals were seen on splenovenography in four of these patients. Hepatic vein wedge pressure was above 7 mmHg in nine patients and these high levels were a result of increased free hepatic vein pressure. The corrected sinusoidal pressure and post sinusoidal resistance were essentially normal in all patients. Evidence of increased pre-sinusoidal resistance was seen in six patients. Estimated hepatic blood flow above 1500 ml/min was seen in six patients and all had evidence of histological portal or sinusoidal infilatration. Patients with 'idiopathic' splenomegaly regardless of increased liver blood flow did not have a significant increase in intrasplenic pressure and no alteration in other haemodynamic parameters. It appears that increased liver blood flow alone in a normal liver can only minimally elevate intrasplenic pressure but can play a significant part in the pathogenesis of portal hypertension when liver structure is altered.