The sinusoidal 8-13 Hz wave form that dominates in the occipital EEG record during relaxed wakefulness is known as alpha rhythm or dominant occipital rhythm. The frequency of this rhythm is known to decrease with normal aging with organic brain diseases. Here we report dominant occipital frequency (DOF) data for 41 early stage AD patients, 50 age-, sex- and health-matched control subjects and 22 subjects with major depressive disorder. DOF was assessed blindly for each subject from 10 or more samples during relaxed wakefulness in an all-night polysomnogram. Mean DOF was significantly lower in the AD (mean +/- S.D. was 8.24 +/- 1.04) than control group (9.48 +/- 0.57; F = 56, P less than 0.001). Mean DOF was also significantly lower in the depressed than control group (9.20 +/- 0.50 vs. 9.48, F = 5.0, P less than 0.05). In discriminant analyses, DOF achieved an overall correct classification rate of 77% for AD vs. control subjects, and 72% for AD vs. depressed subjects. These results indicate that DOF may be useful in identifying early stages of AD in healthy, drug-free populations.