We observed several cases of patients who believed they were free of symptoms or signs of primary hyperparathyroidism (pHPT) preoperatively. reported a change of complaints following parathyroidectomy (PTX). We, therefore, decided to examine a larger group of patients to discover if these findings were incidental or of more general significance. The role of PTX in these patients with asymptomatic pHPT remains controversial. In 1991 criteria were defined at a NIH-consensus conference, according to which patients qualify for either operative therapy or long term medical surveillance. Until now, it was generally believed that the majority of asymptomatic patients would never develop symptoms. In a epidemiological cohort-study, the perioperative data of 582 consecutive patients with pHPT, including 116 asymptomatic patients (20.9%), who underwent parathyroidectomy between 1987 and 1998 were evaluated by uni- and multivariate analysis. At a median of 72 months postoperatively, all patients underwent a planned follow-up which included a standardised, validated questionnaire, physical examination and laboratory investigations. Eighty-six patients who were asymptomatic preoperatively were available for follow-up. Only eight (9.3%) were definitely asymptomatic, 4.6% of the entire, representative cohort. Postoperative improvement was reported in 81.4% of the "asymptomatic" patients. Multivariate analysis did not reveal a single or a set of preoperative measurements, that would allow to predict the retrospectively definitely asymptomatic patient. PTX resulted in normocalcaemia in 98.8% of preoperatively asymptomatic patients, with an operative morbidity of 1.2% and no mortality. Many apparently asymptomatic patients with pHPT will only realise that they did in fact have preoperative symptoms in retrospect, following PTX. This study suggests that using an up-to-date definition of asymptomatic pHPT, there are only a small number of truly asymptomatic patients and that these cannot be predicted preoperatively, as their symptoms may become apparent only after PTX. "Asymptomatic" patients with pHPT may share the same objective and subjective benefits from PTX as symptomatic patients. They should be operated as soon as the diagnosis is established.