Secondary hyperparathyroidism (SHPT) develops as a compensatory mechanism when the body is in calcium deficit. SHPT may be harmful and has been associated with elevated blood pressure. The cause of SHPT could be low calcium intake, reduced intestinal calcium absorption, or increased excretion. However, the relative importance of these factors for the development of SHPT is not known. During the 5th Tromsø study, serum PTH and calcium were measured in 7954 subjects. Then 96 subjects with SHPT (defined as serum PTH above 6.4 pmol/l together with serum calcium below 2.40 mmol/l) and 106 control subjects were examined at follow-up with a food frequency questionnaire, calcium absorption test, measurement of 24-h urinary calcium excretion, and serum vitamin D status. The statistical analyses showed several interactions necessitating subgroup analysis. It was found that the calcium intake was significantly lower in the SHPT group, but only in nonsmoking males; the calcium absorption was nonsignificantly higher in the SHPT group; the serum 25-hydroxyvitamin D levels were significantly lower in the SHPT group but only in nonsmokers; and the 24-h urinary calcium excretion was significantly lower in the SHPT group but only in those not on blood pressure medication. The most frequent cause of SHPT appeared to be low calcium intake (18%) and a low serum 25-hydroxyvitamin D level (18%). However, in most subjects with SHPT all tests were within the normal range, and the cause is therefore probably a combination of several factors.