Abstract The aim of the study was to define clinical interpretation of the parallel measurements of serum fructosamine and HbA 1 in diabetic patients. We studied 14 type 2 diabetic patients over a 16-wk period. The cross-sectional analysis showed no correlation between serum fructosamine and HbA 1 concentrations during the period of changing glycaemic control. The correlations, however, became significant ( P < 0.05) at 12 ( r = 0.60) and 16 ( r = 0.87) weeks, i.e. after glycaemia had stabilised. Longitudinal analysis of individual patients' data over the 16-wk period showed a significant correlation between serum fructosamine and HbA 1 ( r = 0.55 to r = 0.94) which was present in 8 out of 14 patients. The changes in fructosamine concentration preceded those observed in HbA 1. The ratio of fructosamine/HbA 1 predicted the changes in HbA 1 over the following month ( r = 0.54, P < 0.001). Thus, we demonstrated that the parallel measurement of fructosamine and HbA 1 provides information on future trends in HbA 1 concentration in diabetic patients.