A new infertility syndrome has recently been described in Finnish Yorkshire boars. Typical for the syndrome is total akinesia and severe tail malformation of the spermatozoa. Morphometric analysis was performed on semen smears from 20 affected and 18 control boars and on testicular tissue sections from 5 affected and 4 control boars. Semen morphometry revealed that, in affected boars, the length of the sperm tails was only 33% of that of the controls (15.4 microm vs. 47.0 microm, P < 0.0001). Typical for the spermatozoa of affected boars was also an abundant frequency of proximal cytoplasmic droplets (72.4% vs. 6.9%, P < 0.0001), whereas no major sperm-head abnormalities were recorded. In the testicular tissue samples, viewed at light microscopic level, the volume densities of seminiferous tubules or interstitium did not differ. The most characteristic change in the seminiferous epithelium of the affected boars was a reduced number of elongated spermatids. Densities of Sertoli cells and Leydig cells between affected and control boars did not differ. The ultrastructure of testicular tissue from affected boars showed severe alterations in the assembly of the midpiece and tail of the spermatozoa. As well, a typical finding in the seminiferous epithelium of affected boars was conspicuous deposition of lipid droplets. The pathogenesis of this syndrome severely affects spermiogenesis and motility. Spermatozoa have malformed, short tails, which never become motile. This syndrome is not manifested in the structure or function of other ciliated cells in the affected animals.