Numerous tests to detect anti-HBc IgM have been developed and shown to have different degrees of sensitivity and specificity. One of these assays, Corzyme-M (Abbott Laboratories, North Chicago, Ill.), recently became commercially available. The present study was undertaken to evaluate the clinical utility of this anti-HBc IgM test in establishing the diagnosis of acute hepatitis B using sera from a group of 42 prospectively followed individuals who had been exposed to hepatitis B virus. The Corzyme-M test was highly sensitive in detecting recent hepatitis B virus infection. All 30 patients with symptomatic and 12 with asymptomatic acute hepatitis B virus infection developed anti-HBc IgM. However, the timing of sample testing relative to onset of symptoms in symptomatic patients was important, inasmuch as 2 of 23 patients were negative for anti-HBc IgM early in the symptomatic period, although all were HBsAg positive. The duration of anti-HBc IgM positivity after acute infection was variable, ranging from 2 to 134 weeks. In 14% of patients, anti-HBc IgM remained detectable for more than 1 year. From the data, recommendations are given regarding the usefulness of anti-HBc IgM testing in the diagnosis of acute hepatitis B virus infection.