The invention features methods and compositions for diagnosis and treatment of conditions associated with decreased nitric oxide bioavailability, such as a condition associated with elevated arginase activity, using an arginine- and/or arginase-inhibitor based therapy, which therapies include administration of arginine or an arginase inhibitor, either alone or in combination. The invention also contemplates administration of magnesium with arginine, an arginase inhibitor, or with arginine-arginase inhibitor combination therapy. The invention also features methods and compositions for diagnosis, including prognosis, of conditions associated with arginase activity by assessing the ratio of arginine to ornithine in samples from a subject.