Almost half of the hospitalized influenza patients have a chronic disease, which increases the risk for secondary bacterial infections and for adults >65 years influenza is related to high mortality risk. The impact of diabetes mellitus (DM), asthma bronchiale, cardiovascular disease (CVD) and chronic obstructive pulmonary disease (COPD) on the risk of having a low serum phosphatemia (S-P) in addition to influenza is important to investigate as this increases both morbidity and mortality and can be prevented. Hypophosphatemia could be the explanation for reduced chemo-taxis and phagocytosis, which in addition to respiratory function may increase the risk of pneumonia and sepsis. Data for this study was collected from the medical journals retrospectively for 100 patients admitted to the Department of Infectious Diseases during the study period, 1992-94, with the clinical diagnosis influenza out of which seventy-two cases were used in the calculation. Forty-seven percent of the hospitalized influenza patients had a 2.7-fold risk of suffering from DM than of any other chronic disease and an almost significantly doubled risk of having a low S-P level with a chronic disease. The prevalence of hypophosphatemia (S-P<0.70 mmol/l) was high; 13.0% of the women and 15.0% of the men; 34.0% of all patients had S-P<0.82 mmol/l. Men, in contrast to women, showed clinical signs of a secondary bacterial infection more frequently (12/41 and 6/35, respectively). Our study gives indications for an involvement of low S-P with chronic disease.