During many years of clinical practice, I have seen no deeper anguish than that experienced by the families of schizophrenia patients. Sandra Evans {front page, Dec. 13} describes the dailiness of that pain in writing of the Westwoods' life of impossible choices. A similar theme was echoed by Beverly Sills {Parade, Nov. 24}; she tells of feeling too empty to speak after spending a day with her adult mentally ill son. When well-meaning people suggest that they understand, she answers with the loneliest words in the language: ''They cannot understand.''

The love and responsibility of families like the Westwoods extend beyond their own lifetimes. Painful as the pres-ent may be, they live with a deeper dread -- how will our son or daughter be cared for after our death? Will his relentless needs destroy the lives of our other children?

The long-term solution for problems caused by this biochemical brain disease must come from the laboratory, but a fledgling organization is now providing help for some of these problems. PLAN of Maryland and the District is a nonprofit group founded and managed by families. Its purpose is to act in lieu of parents: to see that basic needs are met, that crises are handled and that the quality of clients' lives is preserved as much as possible. Services are provided at cost, and in some cases of financial need no fees are charged. (And, of course, tax-deductible donations are welcomed from anyone.)

Families of the seriously mentally ill are encouraged to call this organization. In my opinion, PLAN represents a ''best buy'' in the business of alleviating human misery. CAROLYN S. MACKENZIE Washington