This study was conducted to gather information on the prevalence of hepatitis A and hepatitis B in the Asir region of southwestern Saudi Arabia and to optimize the use of routine serological tests in the diagnosis. Three serological markers were determined concurrently by enzyme immunoassay for patients with acute hepatitis admitted to a fever hospital and to suspected hepatitis patients at two general hospitals in Abha during 7 January 1987 until 15 May 1989. These markers were the IgM antibody to hepatitis A, the hepatitis B surface antigen, and the IgM antibody to the core antigen of hepatitis B. Results of the serological patterns obtained indicated that hepatitis A afflicted 43% while hepatitis B affected 9% of the patients with acute hepatitis admitted to the fever hospital. In contrast, hepatitis A could only be diagnosed in 5.6% and hepatitis B in 2% of the patients from the other two hospitals. The incidence of hepatitis A peaked in early childhood while that of hepatitis B peaked in middle age. The identification of some patients was uncertain, particularly those with chronic hepatitis who were negative for the hepatitis B core IgM but positive for hepatitis B surface antigen from normal healthy carriers of this antigen, who accounted for about 6% of blood donors in the region during the study period.GA Jamjoom, SK Quli, AK Shenoy, YS Nqer, M Al-Basha, O El-Amin Buluk, MH Abdul Kafi, O Al-Zughaibi, Hepatitis A and Hepatitis B in the Asir Region, Southwestern Saudi Arabia. 1990; 10(4): 429-433 Determination of immunoglobulin M anti-hepatitis A antibody (IgM anti HAV) constitutes a sensitive and specific test for the diagnosis of hepatitis A.1,2 Diagnosis of hepatitis B is more difficult. Originally, the detection of the hepatitis B surface antigen (HB s Ag) was commonly, though mistakenly, relied on as a marker for hepatitis B. However, chronic healthy carriers of this antigen constitute a significant proportion of the population of Saudi Arabia, averaging about 8% (see references 3 and 4 for review). The existence of this chronic healthy carrier state undoubtedly confuses the diagnosis of hepatitis B disease and has necessitated the use of additional markers of virus replication and host immune response for better clarification of the disease activity. These markers include antibody to the surface antigen (anti HB S ), hepatitis B "e" antigen, and the antibody to this antigen (HB e Ag and anti-HB e , respectively), total antibody to the core antigen of hepatitis B (anti HB c ), and IgM antibody against the hepatitis B core antigen (IgM anti HB c ), level of hepatitis B viral DNA polymerase activity, and the amount of hepatitis B DNA. Some of these markers provide useful information on virus replication and the infectivity of the serum (HB e Ag, polymerase, and HB DNA), or on the status of immunity of the patient (anti HB s , anti HB e ). However, the testing of all these markers taxes the capability of the average hospital laboratory. Determination of the IgM anti HB c antibody has been found to cor...