Several features have a prognostic value in adults with acute lymphocytic leukaemia (ALL). However, in about two-thirds of all cases prognosis remains quite variable, with a substantial number of early relapses. This study shows in 118 adult patients who attained complete remission (CR) between 1978 and 1984 that pretreatment serum total lactate dehydrogenase (LDH) activity was inversely correlated with first CR length. The prognostic value of LDH was higher than that of any other features both in univariable and in multivariable analysis. The value was significant in the whole series as well as in patients who lacked other high risk features. Among non-high risk and low-WBC count cases, patients with LDH less than or equal to 500 U/l had a median first CR duration of 27 months, and a projected 5-year relapse free survival of 36%, versus 9 months and 12% of patients with LDH greater than 500 U/l. These results fit well with the results of a study of ALL in children, and suggest that pretreatment serum total LDH activity is an important risk determinant in adult ALL.