Background: Intrauterine insemination with controlled ovarian hyperstimulation is commonly used as first-line treatment for couples with unexplained subfertility. Since such treatment increases the risk of multiple pregnancy, a couple’s chances of achieving an ongoing pregnancy without it should be considered to identify those most likely to benefit from treatment. We aimed to assess the incremental effectiveness of intrauterine insemination with controlled ovarian hyperstimulation compared with expectant management in couples with unexplained subfertility and an intermediate prognosis of a spontaneous ongoing pregnancy. Methods: 253 couples with unexplained subfertility and a 30- 40% probability of a spontaneous ongoing pregnancy within 12 months were randomly assigned either intrauterine insemination with controlled ovarian hyperstimulation for 6 months or expectant management for 6 months. The primary endpoint of this hospital-based study was ongoing pregnancy within 6 months. Analysis was by intention to treat. This trial is registered with the Dutch Trial Register and as an International Standard Randomised Clinical Trial, number ISRCTN72675518. Findings: Of the 253 couples enrolled, 127 were assigned intrauterine insemination with controlled ovarian hyperstimulation and 126 expectant management. In the intervention group, 42 (33% ) women conceived and 29 (23% ) pregnancies were ongoing. In the expectant management group, 40 (32% ) women conceived and 34 (27% ) pregnancies were ongoing (relative risk 0.85, 95% CI 0.63- 1.1). There was one twin pregnancy in each study group, and one woman in the intervention group conceived triplets. Interpretation: A large beneficial effect of intrauterine insemination with controlled ovarian hyperstimulation in couples with unexplained subfertility and an intermediate prognosis can be excluded. Expectant management for 6 months is therefore justified in these couples. Background: Intrauterine insemination with controlled ovarian hyperstimulation is commonly used as first-line treatment for couples with unexplained subfertility. Since such treatment increases the risk of multiple pregnancy, a couple’s chances of achieving an ongoing pregnancy without it should be considered to identify those most likely to benefit from treatment. We aimed to assess the incremental effectiveness of intrauterine insemination with controlled ovarian hyperstimulation compared with expectant management in couples with unexplained subfertility and an intermediate prognosis of a spontaneous ongoing pregnancy. Methods: 253 couples with unexplained subfertility and a 30- 40 % probability of a spontaneous ongoing pregnancy within 12 months were randomly assigned either intrauterine insemination with controlled ovarian hyperstimulation for 6 months or expectant management for 6 months. The primary endpoint of this hospital-based study was ongoing pregnancy within 6 months. Analyst sis was by intention to treat. This trial is registered with the Dutch Trial Register and as an International Standard Randomized Clinical Trial, number ISRCTN72675518. Findings: Of the 253 couples enrolled, 127 were assigned intrauterine insemination with controlled ovarian hyperstimulation and 126 expectant management. In the intervention group, 42 (33%) women conceived and 29 (23%) pregnancies were ongoing. In the expectant management group, 40 (32%) women conceived and 34 (27%) pregnancies were ongoing (relative risk 0.85, 95 % CI 0.63-1.1). There was one twin pregnancy in each study group, and one woman in the intervention group conceived triplets. Interpretation: A large beneficial effect of intrauterine insemination with controlled ovarian hyperstimulation in couples with unexplained subfertility and an intermediate prognosis can be excluded. Expectant management for 6 months is therefore justified in these couples.