Summary. Recent studies have shown that CD38 expressed as a percentage of the antigen positivity can predict prognosis and disease progression in patients with B-cell chronic lymphocytic leukaemia (B-CLL). The present study showed that quantification of CD38 expressed as antibody-binding capacity (ABC) improves the prognostic value of the percentage of CD38 positivity in B-CLL. In a cohort of 81 patients with B-CLL, a level of CD38 expression of ≥ 30% and an ABC value of 250 proved statistically valid cut-off points to predict disease progression (% CD38: P=0·0027; ABC: P < 0·0001). There was a positive and significant correlation between the percentage of CD38 expression and ABC (r=0·7; P < 0·0001). There was a better discrimination of survival using ABC rather than percentage CD38 positivity (P < 0·0001 compared with P=0·0027). Only ABC predicted for survival in patients under 60 years of age (P=0·0076) or with stage A disease (P=0·0195). Both percentage CD38 and ABC discriminated between time to first treatment for all patients but only ABC predicted time to treatment for stage A patients (P=0·0004). In conclusion, CD38 positivity is an important prognostic factor in B-CLL. However, quantification of CD38 is superior to the percentage positivity and should be used clinically in conjunction with other variables of predictive value to identify B-CLL patients that are likely to progress.