I've had elevated alt (75) for 5-6 months and ultrasound showed a mildly enlarged liver (16.6 cm) but no fat. is there a chance this could be serious?

ANY abnormality: of the Liver function has "serious" possibilities and "benign" reasons...for example "glycogen storage disorders" can cause the abnormalities you mention and many of these are benign! You need evaluation by a Board Certified Gastroenterologist! Ask your PCP (Primary Care Provider) for a referral. Hope this helps and good luck! Dr Z