This single case study reports on a 74-year-old woman with 48-hour cycles of mood disturbance for 2 years. Every other day she awakened feeling sad with low energy, decreased appetite, fatigue, diminished enjoyment of normal activities, increased irritability, occasional self-deprecatory thoughts, and difficulty concentrating. On alternative days she was active, outgoing, energetic, and cheerful. Her past history was marked by mild postpartum depressions, not requiring treatment, with each of her six pregnancies, and a more severe depression at age 57, which seemed to respond to Premarin. During the recent 2-year period of mood swings, the patient was given trials of several heterocyclic antidepressant medications, but side effects precluded the use of therapeutic doses or durations of treatment. Reluctantly, the patient agreed to a trial of lithium carbonate. After the second week of lithium treatment, at a level of 0.4 mEq/ml, she reported marked improvement, feeling fine every day without mood fluctuations. After almost 1 year at this blood level, she remains asymptomatic. Thus, the patient's cyclic 48-hour unipolar depression responded dramatically and completely to low doses of lithium carbonate.