The left upper abdomen contains the large and small intestine, the left kidney deep inside, the spleen and the left lobe of the liver. On top of the diaphragm is the left lower lung where pneumonia could develop and pain radiate into the left upper abdomen.
Most of the conditions are more likely to come from the large intestine, like colon cancer, but constipation may also cause pain in that region from gas pains. Ulcerative colitis would be associated with bloody diarrhea and pain in the left abdomen (upper or lower).
It is interesting to note that leukemia could lead to an extreme swelling of the spleen (splenomegaly=enlarged spleen) so that the spleen can reach far down into the left upper abdomen. This swelling can be so excessive that it takes only very little trauma for the spleen to rupture, which when it occurs is a medical emergency. The surgeon has to rush in to remove the spleen and stop the vessels from bleeding.
Back to the bowel: both the small intestine and the large intestine (colon) can close off from twisting or through adhesions from prior abdominal surgery and cause bowel obstruction. Irritable bowel disease and Crohn’s disease can cause pain in the left upper abdomen.
The left kidney can also cause pain in the left upper abdomen with pyelonephritis where pain irradiates along the ureter towards the bladder. With a kidney stone that usually forms in the center of the kidney the pain migrates down as the stone migrates towards the bladder.
These are the most common conditions causing pain in the left abdomen.