We identified spontaneous reactivation of hepatitis B virus (HBV) retrospectively by utilizing serum alanine aminotransferase and HBV DNA in 19 men (79% homosexual), with an estimated annual incidence of 7.3%. In 11 patients, spontaneous reactivation occurred as a single episode and in eight patients, reactivation was recurrent, with two to five episodes each. The mean serum alanine aminotransferase level was elevated over 10-fold at the peak of reactivation. Serum anti-HBc IgM was detected during 73% of the reactivation episodes. Actuarial analysis revealed that reactivation was long lasting with 45% and nearly 20% of episodes continued after 6 and 24 months, respectively. The course of 24 chronic HBV carriers with a negative serum HBV DNA test and normal alanine aminotransferase levels at initial appearance was unremarkable. We could not identify clinical features predictive of reactivation or its resolution. Severe reactivation hepatitis occurred in three patients (10%), with two deaths (6%). None of the patients lost HBsAg. Spontaneous reactivation in chronic hepatitis B can appear variably, persist long term, recur, and be fatal. Therefore, accurate classification of chronic HBV infection requires prolonged observation, and spontaneous reactivation should be considered a variable in therapeutic trials for chronic hepatitis B.