If a lipoma is left untreated for a long time, it will continue growing. The growth happens gradually and there is a possibility that the lump will grow around nerves and blood vessels. For a long time, lipomas won't cause pain but as the lump grows, the fat tissues may start compressing the blood vessels. This can cause pain and discomfort which may be difficult to bear sometimes. This is the usual stage where a person having lipoma decide to visit a doctor.
Besides the pain, leaving lipomas untreated is also not recommended as the treatment of a large-sized lipoma is quite difficult. At this stage, removing a lipoma without damaging the surrounding muscles and tissues is complex. Furthermore, by ignoring a lipoma, you may end up putting your life at risk as a cancerous lump, called liposarcoma, also has similar symptoms. So, even if you don't need to undergo lipoma surgery, it is still important to get diagnosed and identify whether the fatty lump is cancerous or not.