My son has a lump on the back of his hand, looks like in the vein. not a cyst in wrist. moves up and down hands when curling/straightening hand.

Bump on hand?: By your limited description could be a tendon cystic has or ruptured tendon or a cyst. But ONLY IN PERSON examination and opinions will allow a better diagnosis than here on HealthTap...

Ganglion or other: Soft tissue lumps in tendons that move with finger motion are not as common as those that come from the joints but can be identified on the back of the hand. Most of these are benign but there are some types that are more involved and require surgical removal and something to be done with the tendons. Trauma is another cause for swelling in tendons on the back of the hand. Usually a hand surgeon is someone that should evaluate this problem they can tell you whether it needs further treatment or not and typically will take an x-ray and do an examinatiothey also might order and ultrasound to see if the mass is cystic or solid

Ganglion cyst: can occur along any tendon, and although they are most common in the wrist and the palm of the hand, could occur on the back of the hand. Since it moves with clenching the fist, it sounds like it is attached or part of the tendons that move the fingers. A small bruise or hematoma could also cause similar findings. If not painful or enlarging, ok to wait until his routine physical to check it.

Valve of vein?: Hard to tell without seeing it. Could be a cyst or the valve of a vein.

Have you...: Have you made an appointment for your son to be seen by a Physician? Probably no urgency, but it should be visualized.

Lump: Hi The lump sounds consistent with ganglion cyst. If it is getting bigger, painful or the skin above gets red then you may need to be seen.