We investigated the possible value of an in vitro drug-induced cytotoxicity assay in predicting clinical response to fludarabine (FAMP) in chronic lymphocytic leukaemia (CLL). The median FAMP-LD50 values for cases designated as complete response (CR), partial response (PR), no response (NR) and progressive disease (PD) were 1.43, 2.05, 24.8 and 77.9 microg/ml, respectively (P=0.013). A significant lower mean FAMP-LD50 value was observed in the in vivo responding cases as compared to non-responding ones (3.0 +/- 0.82 SEM v 42.3 +/- 12.6 SEM, P=0.001). Only FAMP-LD50 < or = 2.5 microg/ml and more than four FAMP courses significantly influenced the response rate in univariate and multivariate analyses. The MTT assay may be of value in predicting clinical response to FAMP in CLL patients.