Strict criteria are clinically helpful and can be used to counsel patients and to plan correct treatment in assisted reproduction to alleviate male-factor problems. It can be concluded that normal sperm morphology is needed for normal sperm function, especially sperm-zona pellucida binding, the acrosome reaction with release of (pro)acrosin, and sperm-oolemma fusion. The inclusion of an acrosome index, however, can be a more valuable addition to the evaluation of sperm morphology for the prediction of expected IVF rates, especially in patients with 4% or less morphologically normal spermatozoa.