What else might be causing your cat to lose his hair? It could be a food allergy. If you've changed your cat's food lately, simply changing it back could get rid of the problem. Cats can also develop food allergies over time, so you may need to try different foods.
Does your cat have a "hot spot"? Also known as acute moist dermatitis, this condition is more common in dogs, but cats can get it, too. Hot spots start with a skin irritation, such as a flea or tick bite. When your cat grooms the area, bacteria can infect the wound. These oozing sores are hot, painful, and can spread quickly. Your cat makes the problem worse by licking the area, keeping it wet and preventing it from healing. After diagnosing and treating the underlying cause (if there is one), the hot spot needs to be treated. Your veterinarian may shave around the area to promote drying and prescribe antibiotics.
Hair loss can also be self-inflicted due to psychological factors like stress; upset cats will sometimes groom excessively to soothe themselves. When you see your cat doing this, you can try to redirect the behavior by giving him more attention (or, ideally, removing the source of stress, although that's not always possible). Your cat may also benefit from anti-anxiety medication; consult with your vet if the problem becomes severe.
If the alopecia is in a hard-to-reach place (between the shoulder blades, for example), it's probably not self-inflicted. One possibility is contact dermatitis -- an allergic reaction from something your cat has rubbed against. If the hair loss is limited to the lower legs, for example, it could be a reaction to a new rug. You may have to do some detective work to figure out the cause, but once you remove it, the hair should grow back. Your vet may prescribe a hydrocortisone spray to stop the cat from scratching.
Alopecia can also signal a more serious problem, such as a hormone imbalance. Cats with either hypo- or hyperthyroidism (an under- or over-active thyroid) may experience patchy hair loss, and an excess of the hormone cortisol can cause thinning hair in a symmetrical pattern on your cat's trunk. These conditions must be diagnosed with a blood test. If your cat's hair loss is accompanied by changes in appetite, vomiting, weight loss or fever, get him to the vet immediately.