BACKGROUND Telbivudine (LdT) is an L-nucleoside that is structurally related to lamivudine. It is highly selective for hepatitis B virus (HBV) and inhibits viral DNA synthesis. LdT was approved by the US Food and Drug Administration on October 25, 2006, for the treatment of chronic HBV infection in adults who have active viral replication and either elevations in liver transaminases or signs of active liver disease on histologic examination. OBJECTIVE This article reviews the pharmacology, pharmacokinetics, and therapeutic efficacy of LdT. Potential drug interactions and adverse events associated with the use of LdT are also reviewed. METHODS Relevant publications were identified from searches of MEDLINE (1996-June 2007), the Cochrane Library, and BIOSIS (1993-June 2007). Search terms included, but were not limited to, telbivudine, beta-L-thymidine, LdT, pharmacology, pharmacokinetics, adverse events, resistance, drug interactions, hepatitis B, and therapeutic use. Additional publications were identified from the reference lists of the identified papers, meeting abstracts, and correspondence with the manufacturer of LdT. RESULTS After 52 weeks of therapy in the Phase III GLOBE study, HBV resistance (breakthrough and resistance mutations) to LdT occurred in 3% of patients who were hepatitis B e antigen (HBeAg) positive and 2% of patients who were HBeAg negative. After 104 weeks of therapy, 17.8% to 21.6% of HBeAg-positive and 7.3% to 8.6% of HBeAg-negative LdT-treated patients had a rebound in HBV DNA associated with breakthrough and resistance mutations. After 24 weeks of treatment, the risk of resistance was greater in patients with HBV DNA titers >3 log(10) copies/mL than in those with lower numbers of copies. LdT is not active against lamivudine-resistant HBV. The proportion of HBeAg-positive patients with undetectable HBV DNA (by polymerase chain reaction assay) after 104 weeks of therapy in the GLOBE study was significantly greater with LdT compared with lamivudine (56% vs 39%, respectively; P < 0.05). After 104 weeks of therapy, the corresponding proportions of HBeAg-negative patients with undetectable HBV DNA were 82% and 57% (P < 0.05). Patients who failed lamivudine therapy in the GLOBE study showed cross-resistance to LdT. The most common adverse events associated with LdT are upper respiratory tract infection (14%-17%), fatigue and malaise (12%-14%), nasopharyngitis (11%-15%), headache (11%-12%), and abdominal pain (6%-12%). Grade 3/4 adverse events included elevations in serum creatine kinase, which were more common in patients receiving LdT than in those receiving lamivudine (9% vs 3%, respectively). Elevations in creatine kinase are typically asymptomatic; however, myopathy has been reported in 3 of 680 patients receiving LdT. CONCLUSIONS LdT joins the increasing number of antiviral agents for the management of chronic HBV infection. Questions concerning the optimal length of therapy and long-term efficacy await the results of on-going clinical trials. Concerns about increasing resistance over time may relegate LdT to second-line status in the management of chronic HBV infection. The role of LdT in combination therapy is under investigation.