In this paper, dive computer-generated no decompression limits (NDLs) in fresh water at high altitude were compared with low-altitude single, repetitive and multilevel dives. All computer-generated high-altitude NDLs exceeded those published for the altitude dived. Computer rankings by conservatism for single dives at low altitude had negative correlation with rankings at high altitude (r = −0.81). Correlation between high-altitude square-profile dives and low-altitude repetitive, multilevel NDLs was significantly higher (r = 0.91, p <0.01). We conclude sea-level single-dive NDLs, such as those published in instruction manuals, are not reliable when gauging the conservatism of dive computers for use at high altitude. It is recommended that divers using dive computers for planning high-altitude dives to consider computer-generated real-time NDLs as experimental.