To study the relation between beta 2 microglobulin (beta 2M) and survival in AL amyloidosis, we measured the serum level of beta 2M in 80 patients with AL amyloidosis diagnosed within 1 year of evaluation, who had received no therapy. Patients had a median age of 61 years and 52% were male. Major clinical manifestations were renal disease in 25 patients (31%), cardiomyopathy in 23 patients (29%), and neuropathy or other organ involvement in 32 patients (41%). The beta 2M level, measured by an ELISA assay in serum samples collected at the time of evaluation, ranged from 1.69 to 10 mg/ml (mean = 4.57); in 56% of the patients beta 2M > 4 mg/ml. The patients with a beta 2M < or = 4 mg/ml had serum creatinine levels lower than those with beta 2M > 4 (1.43 vs 2.67 mg/dl; p = 0.02). Survival from study entry was analyzed overall by the level of beta 2M, adjusting for creatinine level and clinical stratum. We found the beta 2M level to be predictive of survival (median survival 16.1 months for beta 2M < or = 4 mg/ml vs 8.0 months for beta 2M > 4 mg/ml, p = 0.044). Thus a beta 2M level less than 4 mg/ml indicated a longer time of survival.