Abstract: Program comprehension is a fundamental activity in software development that cannot be easily measured, as it is performed inside the human brain. Using a wearable Near Infra-red Spectroscopy (NIRS) device to measure cerebral blood flow, this paper tries to answer the question: Can the measurement of brain blood-flow quantify programmers' mental workload during program comprehension activities? We performed a controlled experiment with 10 subjects; 8 of them showed high cerebral blood flow while understanding strongly obfuscated programs (requiring high mental workload). This suggests the possibility of using NIRS to measure the mental workload of a person during software development activities.