Background Risk factors for loss of virological response in patients receiving lopinavir/ritonavir (LPV/r) monotherapy as maintenance treatment have not been determined. Methods In 121 patients enrolled in the OK and OK04 clinical trials assigned to receive monotherapy with LPV/r, we attempted to identify factors associated with loss of virological suppression at 48 weeks, defined as confirmed serum HIV type-1 RNA>50 copies/ml, with missing data or changes caused by toxicity censored. Univariate and multivariate Cox proportional hazard models were used to calculate hazard ratios for the risk of loss of virological suppression. Results At week 48, 15 patients experienced loss of virological suppression. Probability of loss of virological suppression was 12.7%. Less than 9 months of maintenance of virological suppression prior to monotherapy, a lower baseline haemoglobin and low adherence measured by self-reported total missed doses in the week prior to study visit were associated with loss of virological suppression in the univariate analyses. Independent factors associated with loss of virological suppression by multivariate analyses were ≥2 visits with self-reported missed doses in the week prior to the study visit, a lower baseline haemoglobin and a nadir CD4⁺ T-cell count <100 cells/μl Conclusions Suboptimal adherence, lower baseline haemoglobin and a nadir CD4⁺ T-cell count <100 cells/μl were the main risk factors for losing virological suppression in patients randomized to monotherapy with LPV/r.