Background The cost-effectiveness of routine viral load (VL) monitoring of HIV-infected patients on antiretroviral therapy (ART) depends on various factors that differ between settings and across time. Low-cost point-of-care (POC) tests for VL are in development and may make routine VL monitoring affordable in resource-limited settings. We developed a software tool to study the cost-effectiveness of switching to second-line ART with different monitoring strategies, and focused on POC-VL monitoring. Methods We used a mathematical model to simulate cohorts of patients from start of ART until death. We modeled 13 strategies (no 2nd-line, clinical, CD4 (with or without targeted VL), POC-VL, and laboratory-based VL monitoring, with different frequencies). We included a scenario with identical failure rates across strategies, and one in which routine VL monitoring reduces the risk of failure. We compared lifetime costs and averted disability-adjusted life-years (DALYs). We calculated incremental cost-effectiveness ratios (ICER). We developed an Excel tool to update the results of the model for varying unit costs and cohort characteristics, and conducted several sensitivity analyses varying the input costs. Results Introducing 2nd-line ART had an ICER of US 1651-1766/DALYaverted.Comparedwithclinicalmonitoring,theICERofCD4monitoringwasUS 1896-US 5488/DALYavertedandVLmonitoringUS 951-US 5813/DALYaverted.WefoundnodifferencebetweenPOC-andlaboratory-basedVLmonitoring,exceptforthehighestmeasurementfrequency(every6months),wherelaboratory-basedtestingwasmoreeffective.TargetedVLmonitoringwasonthecost-effectivenessfrontieronlyifthedifferencebetween1st-and2nd-linecostsremainedlarge,andifweassumedthatroutineVLmonitoringdoesnotpreventfailure.ConclusionComparedwiththelessexpensivestrategies,thecost-effectivenessofroutineVLmonitoringessentiallydependsonthecostof2nd-lineART.OurExceltoolisusefulfordeterminingoptimalmonitoringstrategiesforspecificsettings,withspecificsex-andage-distributionsandunitcosts.