Temporal interictal rhythmic delta activity or TIRDA was found in 45 of the 127 recordings of patients with complex partial epilepsy (CPE) referred for both awake and sleep EEG. TIRDA was more abundant during drowsiness and light sleep; it occurred more characteristically as trains of 50-100 microv sinusoidal or saw-toothed 1-4Hz activity, recorded predominantly from anterior temporal regions. When occurring bilaterally and independently, TIRDA varied from side to side. TIRDA is often found in association with anterior temporal spikes or sharp waves (TS) particularly during sleep, as observed in 43 out of 45 EEGs. TIRDA can nevertheless occur as an isolated abnormality, as noted in two sleep and 12 awake study recordings. Because of its high specificity and positive predictive value over a large range of prevalence, TIRDA should be singled out as an accurate interictal indicator of CPE. In patients with isolated TIRDA, the cost of prolonged EEG recording during sleep for the purpose of activating TS has to be weighed against the yield of eventually confirming the obvious.