Montreal, QC, Canada
Based upon your very brief description, I suspect your cat is suffering from psychogenic alopecia, or feline trichotillomania. Many experts believe that the syndrome is related to obsessive compulsive disorder in people.
Cats with psychogenic alopecia groom themselves excessively. This leads to bald patches on the skin. The overgrooming may occur in response to stress, boredom, or mild allergic stimulation from fleas or other allergens. The bald patches most often start on the abdomen and then spread to the rear aspects of the thighs, then up the back from the base of the tail.
Any person who suspects her cat is suffering from psychogenic alopecia should see a vet to confirm the diagnosis and rule out medical causes of hair loss such as severe allergies, parasite infestation, and autoimmune disease.
Cats with psychogenic alopecia may benefit from a stable home environment and increased enrichment (this can be accomplished through training, increased activity, and altered feeding tactics). In some cases medications such as antidepressants and rarely estrogen analogues are prescribed to reduce symptoms. In general I do not recommend medicating cats with psychogenic alopecia (I especially recommend against estrogen analogues). Psychogenic alopecia is a cosmetic problem, and I generally feel that it does not warrant systemic medication.
Photo: psychogenic alopecia may start out as a bald patch on the abdomen.