['To find the underlying defect in a case of primary FSH deficiency and to estimate the beneficial effect of FSH treatment.', 'Case report.', 'University hospital fertility clinic.', 'Normal, healthy, 37-year-old male patient with severe oligoteratozoospermia.', 'Levels of FSH, LH, LHRH provocation test, karyotyping, genomic analysis on the Y-chromosomal AZF region and sequencing of the FSHB gene, FSH treatment.', 'We compiled detailed clinical and molecular data on four pregnancies. We compare this case with a similar case published recently.', 'There were detectable but very low FSH levels after LHRH provocation; the LH response was not entirely normal, and no genomic abnormalities were found in the FSHB gene. The FSH treatment resulted in four pregnancies, two of which ended in abortion; the other two resulted in the birth of two healthy children. Both our case and the published case had detectable but abnormally low FSH levels on some occasions, but normal or highly normal inhibin B levels that differed from the expected low levels. Both patients had a normal male phenotype and no detectable mutation in the FSHB gene. The published case differed from our patient in that the published case was azoospermic whereas ours was extremely oligoteratozoospermic. The beneficial effect of FSH treatment was only shown in our patient.'] Is apparent primary follicle-stimulating hormone deficiency a rare cause of treatable male infertility?