Beta 2-microglobulin predicts survival in primary systemic amyloidosis. PURPOSE: The study assessed whether beta 2-microglobulin levels predict survival or response in patients with primary systemic amyloidosis without associated multiple myeloma. PATIENTS AND METHODS: The study group consisted of 131 untreated patients with biopsy-proven primary systemic amyloidosis diagnosed and evaluated at the Mayo Clinic. No patient had multiple myeloma. The minimum follow-up of surviving patients is 8 years. No patient was lost to follow-up. RESULTS: The median survival of patients with an increased beta 2-microglobulin level was 10.8 months, compared with patients with a normal beta 2-microglobulin level (less than or equal to 2.7 micrograms/mL, 0.23 mumol/L) of 32.9 months (p less than 0.001). In a multivariate proportional-hazards model, the best model included congestive heart failure (p less than 0.0001) and increased beta 2-microglobulin levels (p less than 0.05). After adjustment for the presence of congestive heart failure, beta 2-microglobulin level remained significant. When the analysis was restricted to those patients with normal renal function, the median survival of those with an elevated beta 2-microglobulin level was 9.1 months versus 39.4 months for those with a normal level (p less than 0.001). The serum level of beta 2-microglobulin was increased in patients with nephrotic-range proteinuria with or without renal insufficiency (p = 0.05). CONCLUSION: The serum beta 2-microglobulin level should be measured routinely in all patients with primary systemic amyloidosis because it provides a useful objective factor to identify subsets of patients with this disease who have unfavorable outcomes. 