Serum levels of beta 2-microglobulin (S-B2m) were analysed at diagnosis in 69 cases of acute myeloid leukemias (AML) as a possible prognostic indicator. S-B2m was correlated to various clinical and laboratory features and with respect to response to chemotherapy and following clinical outcome. S-B2m was found to be increased (greater than 3 mg/l) in 40.6% of tested patients and, in particular, in the monocytic variants (M4, M5) of AML (4.2 versus 2.3 mg/l, p less than 0.01). S-B-B2m values paralleled white blood cell count, serum lysozyme levels and expression of monocytic membrane markers at presentation, but no correlation was found with age, renal function or immunological myeloid antigens. Increased levels of S-B2m were associated with a lower likelihood of obtaining a complete remission (25 versus 58.5%, p less than 0.01), while in the multivariate analysis S-B2m greater than 3 mg/l and white blood cell count greater than 20 x 10(9)/l were independent variables significantly influencing disease-free survival in responsive patients (five years DFS for S-B2m greater than or less than 3 mg/l: 28 versus 62%, p less than 0.05). In conclusion, the measurement of S-B2m at diagnosis may have prognostic relevance in AML.