Yes, the thickness of the mattress is important. A bunk bed mattress should only be used on a bunk where the guardrail is at least 4-inches above the mattress surface. Five inches would be better, but many bunk beds have high guard rails. We have included one mattress at 10-inches thick that is suitable only for a bunk bed with guard rails 14 inches high. This has been made clear in the review. However, it is a great mattress for anybody with a bunk bed with that specification.