Histologically confirmed, clinically asymptomatic viral hepatitis was found by serial surveillance of the serum glutamic pyruvic transaminase (SGPT) levels in 4 of 478 US soldiers given prophylactic γ-globulin in a hepatitis-endemic area of Thailand. This is an incidence of 15.4 cases/1,000 man-years at risk in this environment. Biopsy in 21 other subjects with an SGPT level of 50 or more units revealed nonspecific alterations in 8, fatty metamorphosis in 8, and normal liver in 5. Of the total of 25 subjects with biopsies, 80% had histologically abnormal findings. Those with biopsyconfirmed hepatitis had SGPT elevations for at least four weeks with a peak level of 500 or more units. Thus, correlation between histologic findings and magnitude and duration of SGPT abnormality was good.