The prevalence of antibodies to hepatitis A (anti-HA) was investigated in patients attending a clinic for haematological malignancies. Patients were divided into three age groups; the number with antibodies in each group was recorded. Of the 130 patients studied, 44% of those aged 4-26, 70% of those aged 27-50, and 83% of those aged 51-76, were positive. This study identifies patients attending a clinic for haematological malignancies (ie, patients with immunologic deficiencies) as a group of people having a high prevalence of anti-HA. In the oldest group (51-76), urban patients had a higher prevalence than rural patients - a pattern similar to that observed in elderly Red Cross blood donors in a previous study. Seroconversion was noted in six patients, three of whom seroconverted from anti-HA-negative to anti HA-positive within 12 months of the initial visit to the clinic.