Major monomorphous teratozoospermia, due to sperm tail structural anomalies, were detected in 42 out of 4231 infertile patients during routine semen analysis. The flagella were very short or absent in 16 cases, shortened with thickness irregularity in 18 cases, and of normal length with diameter anomalies in eight cases. These syndromes were always associated with poor forward motility, while mobility and penetration into human cervical mucus were sometimes impaired only partially. With good sperm smears and an experienced observer, the diagnosis could be made without the need for electron microscopic analysis. All the affected patients were sterile and several syndromes could have been transmitted genetically.