Thirlwall's law, given by the ratio of the rate of growth of exports to the income elasticity of imports is a key result of balance-of-payments-constrained long-run growth models with balanced trade. Some authors have extended the analysis to incorporate long-run net capital flows. We provide a critical evaluation of these efforts and propose an alternative approach to deal with long-run external debt sustainability, based on two key features. First, we treat the external debt-to-exports ratio as the relevant indicator for the analysis of external debt sustainability. Second, we include an external credit constraint in the form of a maximum acceptable level of this ratio. The main results that emerge are that sustainable long-run capital flows can positively affect the long-run level of output, but not the rate of growth compatible with the balance-of-payments constraint, as exports must ultimately tend to grow at the same rate as imports. Therefore, Thirlwall's law still holds.