Recent studies have strongly indicated that at least three regions [azoospermia factor (AZF) a-c] on the long arm of the Y-chromosome code for factors involved in spermatogenesis. In order to reveal the prevalence of microdeletions in these regions in a Swedish population, 192 men consecutively referred to our andrology unit due to infertility and showing oligozoospermia (n=53) or azoospermia (n=139) but no obstruction or hormonal disturbances, were investigated. For this study we used a multiplex polymerase chain reaction (PCR) method including 13 pairs of primers divided into five different primer mixes. It was found that four men, all with azoospermia, had deletions including part of the AZFb region and probably the entire AZFc region. Testis biopsies showed different morphology ranging from absence of germ cells to hypospermatogenisis. Of special interest was one patient that was first investigated 10 years ago due to primary infertility and oligozoospermia. Today he has developed azoospermia. It is concluded that the number of patients with microdeletions on the Y chromosome is rather low (less than 3% in highly selected azoospermic men) in our study compared to a number of other studies in which a 1-55% incidence have been reported. It is possible that ethnic differences, selection criteria and methodological aspects can contribute to the difference between the present and previous studies.