BACKGROUND: Mild hypothyroidism (thyroid stimulating hormone [TSH] less than 10 mIU/L) induces reversible cognitive dysfunction, which can be evaluated by event related potentials (ERP). So far, only little is known about the impact of subclinical hypothyroidism on ERP as electrophysiological markers of cognitive activity. The aim of this study was to follow-up P300 latencies and amplitudes in patients with subclinical hypothyroidism and to evaluate the influence of thyroxine treatment which led to the normalization of TSH level in serum. METHODS: We recorded the P300 wave using an auditory oddball paradigm in 60 patients (mean age 5 1.1 +/- 6.2 years, range 40-62 years), with subclinical hypothyroidism (normal mean value of FT4, with elevated TSH levels) at baseline, after 3 months, after 6 months and in 30 healthy control subjects. 30 patients treated six months with L-thyroxine until the normalization of TSH and 30 patients received placebo. RESULTS: The P300 latencies in patients with subclinical hypothyroidism were significantly longer, and the P300 amplitudes were significantly smaller than those of the control group. In the thyroxine treated patients P300 latency continuously decreased over the observation period with a significant difference after 6 months compared to baseline (P<0.01). The amplitude P300 showed no significant changes over time. CONCLUSIONS: Our results show the importance of P300 event related potentials in the detection of cognitive changes in patients with hypothyroidism. The P300 latency stands out as a marker for cognitive function recovery during treatment with thyroxine.