Bacterial infections were registered in 39 patients with myelomatosis during 18 months in a prospective study. The infection incidence was 0.80 infections per patient year. 81% of a total of 32 isolates were gram-negative. Urinary tract infections due to Escherichia coli were the most frequent infections. Pneumonia due to Streptococcus pneumoniae were infrequently seen compared to previous studies. Hence, the etiologic spectrum has clearly shifted from gram-positive to gram-negative bacteria in these patients. 53% of all infections were hospital-acquired, and most of these were preceded by instrumentation of the urinary tract or indwelling venous catheters. The infections were nosocomial in 7/9 cases of septicemia registered. All 4 patients who died of infection, suffered from hospital-acquired infections. Patients who attracted infections had significantly higher serum creatinine levels and higher mortality compared to the rest of the patients.