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Not much is known about what causes lung carcinoid tumors. Researchers have learned a lot about how certain risk factors like cancer-causing chemicals or radiation can cause changes in lung cells that lead to carcinomas, the more common type of lung cancer. But these factors are not thought to play a large role in the development of lung carcinoid tumors.

Carcinoid tumors probably develop from tiny clusters of neuroendocrine cells called carcinoid tumorlets in the lung airways. Tumorlets are sometimes found unexpectedly in lung biopsies done to treat or diagnose other conditions. In the lab, tumorlets resemble carcinoid tumors, except that they are much smaller – less than 5 mm (about ? inch) across. Most tumorlets never grow any bigger, but some may eventually become carcinoid tumors.

If tumorlets are found throughout the lung along with an overgrowth of neuroendocrine cells, this is called diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (DIPNECH). If DIPNECH is found on a biopsy sample this may mean a lung carcinoid might develop, but lung carcinoids can also develop without DIPNECH being present.

Researchers have found some common changes in chromosomes and genes inside lung carcinoid tumor cells, which might affect how these cells function. But it is still not clear exactly how these changes cause carcinoid tumorlets to develop from lung neuroendocrine cells or how they might cause tumorlets to grow and become carcinoid tumors.