Smelly scalp syndrom, like dandruff, is caused by scalp-specific fungus. I would recommend using shampoo that contains tea tree oil. Choose your brand carefully, and if you already use an all-natural shampoo that you are satisfied with, you could also add 1 tsp. of tea tree oil to a 32 oz. bottle of shampoo for the same effect. If you have tried tea trea oil shampoos and other methods and the side effects do not subside, it may be time to consult a doctor who can prescribe something for you (this may not necessarily be a "natural" option, but you may get lucky with a recommended homeopathic alternative from your doctor). Refer to the two links below for information on both tea tree oil and smelly scalp syndrome.