The concentration of thiamine (vitamin B1) was measured in blood cells and plasma from mothers with normal pregnancy and from mothers whose pregnancy was complicated by intrauterine growth retardation (IUGR). Thiamine concentrations were estimated by the thiochrome method using HPLC separation and fluorimetric detection according to Weber et al. (2). During normal pregnancies the thiamine values in blood cells fall in the 28th to the 39th week of gestation from 230 nmol/l to 170 nmol/l. In cases with severe IUGR there is only a slight decrease in the thiamine levels from 140 nmol/l in the 30th week of gestation to a level of 130 nmol/l in the 39th week of gestation. During this period the thiamine values in plasma remain constant. To compare the thiamine concentrations in normal pregnancies and in those with IUGR we separated the thiamine values in a group from 28/0 to 34/6 and one from 35/0 to 39/6 weeks of gestation. In both groups mothers with normal pregnancy had significantly higher thiamine levels in the blood cells than mothers whose pregnancy was complicated by IUGR (p = 0.0001 and p = 0.0005). However, the thiamine values in plasma were not significantly different in normal pregnancies and pregnancies with IUGR. The results indicate that maternal thiamine deficiency might be one cause of IUGR.