AIM: Predictive value of serum b2-microglobulin (b2m)levels for virological breakthrough (VB) in HBeAg-negative chronic hepatitis B (CHB) patients under long-term treatment schedules including lamivudine (LAM).METHODS: Serum b2m levels were calculated during treatment in 25 CHB patients under long-term LAM monotherapy (group A) and 12 patients under initial interferon plus LAM treatment followed by LAM monotherapy (group B), using the MEIA technology. We used Cox proportional hazard models in order to investigate the association between serum b2m levels and VB.RESULTS: Seven of 25 patients (28%), 9/25 (36%) and 14/25 (56%) from group A and 0/12, 2/12 (16.6%) and 3/12 (25%) from group B exhibited VB at months 12, 24 and 36 of treatment, respectively. All patients, from both groups, who did not show VB exhibited b2m elevation in mo 3. The duration of b2m elevation was significantly longer in the virological responders subgroup from group A than the non-responders one (7.3±2.6 vs 3.8±3.4 mo,P = 0.02). In comparison to group A patients whose b2m levels were increased at 3 mo, patients whose b2m levels were decreased had 4.6 times higher risk of experiencing VB (RR = 4.6, P = 0.024). When baseline variables were simultaneously included in the same Cox model, decreased b2m status was still associated with increased risk of VB (RR = 12.2, P = 0.03).CONCLUSION: In HBeAg-negative CHB patients under either long-term LAM monotherapy or initial combination treatment, serum b2m levels at 3 mo of treatment,compared to baseline ones, might be a predictor of risk for VB.