Our aim was to evaluate whether the serum level of galectin-1 (Gal-1) at 18-24 and 27-31 weeks of gestation is a risk factor for predicting the later occurrence of not only preeclampsia (PE) but also gestational hypertension (GH). We measured serum levels of soluble fms-like tyrosine kinase 1 (sFlt-1), placental growth factor (PlGF), and Gal-1 using an enzyme-linked immunosorbent assay in 81 and 73 normal pregnant women, 22 and 16 women with a later onset of GH, and 37 and 29 women with a later onset of PE at 18-24 and 27-31 weeks, respectively. We also measured Gal-1 in 33 women with GH and 78 women with PE after the onset. The levels of Gal-1 after the onset of GH, late-onset PE (onset at ⩾34 weeks), and early-onset PE (onset at <34 weeks) were significantly higher than those in normal pregnant women at 27-31 weeks. However, the low levels of Gal-1 (<8.1 ng ml-1) at 18-24 weeks, but not at 27-31 weeks, predicted the later occurrence of not only early-onset PE and late-onset PE but also GH. The low level of Gal-1 at 18-24 weeks was an independent risk factor for the later occurrence of GH and PE, after adjusting for the effects of a high BP and increased sFlt-1/PlGF ratio at 18-24 weeks. In conclusion, the serum level of Gal-1 is a novel risk factor for both GH and PE, specifically its expression at a low level in the second trimester and a high level after onset.