ObjectivesTo determine the feasibility and acceptability of conducting a randomised trial on the effects of myo-inositol in preventing gestational diabetes in high-risk pregnant women.DesignA multicentre, double-blind, placebo-controlled, pilot randomised trial with nested qualitative evaluation.SettingFive inner city UK National Health Service hospitalsParticipantsMultiethnic pregnant women at 12⁺⁰and 15⁺⁶weeks’ gestation with risk factors for gestational diabetes.Interventions2 g of myo-inositol or placebo, both included 200 µg folic acid, twice daily until delivery.Primary outcome measuresRates of recruitment, randomisation, adherence and follow-up.Secondary outcome measuresGlycaemic indices (including homoeostatic model assessment-insulin resistance HOMA-IR), gestational diabetes (diagnosed using oral glucose tolerance test at 28 weeks and by delivery), maternal, perinatal outcomes, acceptability of intervention and costs.ResultsOf the 1326 women screened, 58% (773/1326) were potentially eligible, and 27% (205/773) were recruited. We randomised 97% (198/205) of all recruited women (99 each in intervention and placebo arms) and ascertained outcomes in 90% of women (178/198) by delivery. The mean adherence was 52% (SD 44) at 28 weeks’ and 34% (SD 41) at 36 weeks’ gestation. HOMA-IR and serum insulin levels were lower in the myo-inositol vs placebo arm (mean difference −0.6, 95% CI −1.2 to 0.0 and −2.69, 95% CI −5.26 to −0.18, respectively). The study procedures were acceptable to women and healthcare professionals. Women who perceived themselves at high risk of gestational diabetes were more likely to participate and adhere to the intervention. The powder form of myo-inositol and placebo, along with nausea in pregnancy were key barriers to adherence.ConclusionsA future trial on myo-inositol versus placebo to prevent gestational diabetes is feasible. The intervention will need to be delivered in a non-powder form to improve adherence. There is a signal for efficacy in reducing insulin resistance in pregnancy with myo-inositol.Trial registration numberISRCTN48872100.