In a prospective study sera from 140 patients with symptoms suggesting a post-viral syndrome and sera from 100 controls were tested for neutralizing antibodies to Coxsackie B viruses. Sixty-five of the patients (46%) and 25 of the controls (25%) had significant antibody titres. The 65 positive cases who had presented with symptoms were followed up and retested six months later and again after one year. Of these 65 patients 36 (55%) were still unwell after one year and high antibody titres persisted in all but two of the patients. Recovery was not found to correlate with a fall in antibody level, but was more rapid in patients whose presenting symptoms were paraesthesiae, anorexia or dyspnoea. The importance of correctly identifying patients with the post-viral syndrome, who may otherwise be labelled neurotic, is emphasized.