Sperm morphology of three healthy boars and three boars with spontaneous abdominal cryptorchidism in the right testis has been evaluated by light microscopy. For each boar, two ejaculates have been analysed, corresponding to semen collections at the ages of 6.5 months (first collection) and 8 months (seventh collection). A comparative study of the sperm malformations present in the seventh semen collection between the healthy boars and the unilateral abdominal cryptorchid boars has also been performed by light microscopy. Sperm malformations of the seventh semen collection from the unilateral abdominal cryptorchid boars have been examined by scanning and transmission electron microscopy. The frequency of mature spermatozoa, immature spermatozoa, aberrant spermatozoa and detached heads maintained normals values in the first and the seventh semen collection from the unilateral abdominal cryptorchid boars. The comparative study of sperm abnormalities in the seventh semen collection between the cryptorchid boars and the healthy boars indicated that the unilateral abdominal cryptorchid boars had a significantly higher frequency of primary abnormalities, and a significantly lower frequency of secondary abnormalities. Some primary abnormalities, such as crater defect, knobbed acrosome defect, nuclear crests and abaxial tails were only observed in the unilateral abdominal cryptorchid boars. It was concluded that unilateral abdominal cryptorchidism provokes disturbances in the late stages of spermiogenesis, at testicular level. Alterations in the sperm maturation process at epididymal level were not found.