In a prospective study of South African Indian subjects with IGT, glycosylated hemoglobin [specifically HbA1 (HbA1(a+b+c)] and its relationship to the oral glucose tolerance test (OGTT) was studied in 128 study subjects who were classified IGT a year previously (Year 0 of study) and in 64 control subjects. At Year 1 of the study, the standard 75-g OGTT was performed on all subjects; study subjects were further divided into three groups based on World Health Organisation criteria [Normal (N), impaired glucose tolerance (IGT), diabetes mellitus (D)]. HbA1, a glycosylated hemoglobin (GHb), was measured by a cation-exchange microchromatographic method. Based on OGTT results, 47 of the 128 study subjects were classified IGT, 41 diabetes (newly-diagnosed diabetes) and 40 subjects had normal glucose tolerance. Mean GHb was significantly higher in the D group (7.61 +/- 1.76%) compared to the control group (6.99 +/- 1.22%) and the N group (6.9 +/- 1.12%), respectively (P less than 0.05); there was no significant difference between the IGT group (7.48 +/- 1.44%) and each of the other three groups. Compared to the OGTT, GHb was relatively insensitive in the diagnosis of IGT or diabetes mellitus: only 17% of the IGT group and 26.8% of the D group has elevated GHb values; the specificity of GHb as a measure of normal glucose tolerance was 85.9%. The majority of subjects, irrespective of the category of glucose tolerance, had GHb levels within the normal range and there was marked overlap between the four groups.(ABSTRACT TRUNCATED AT 250 WORDS)