In a cross sectional field study concerning the male reproductive system in metalworkers, the major findings were a moderate deterioration of semen quality in mild steel welders and less reliable changes in semen quality in low exposed stainless steel welders. In the present study, a longitudinal design was adopted to deal with methodological drawbacks inherent in the cross sectional approach. The study relies on the assumption that the effect of welding is causal and reversible. The semen quality of 19 mild steel welders, 18 stainless steel welders and 16 non-welding metal-workers, was examined before and three, five, and eight weeks after a three week break in exposure (summer vacation). No consistent improvement in any semen parameter in the follow up period relative to the prevacation period was found in either mild steel or stainless steel welders. The results indicate either a non-causal nature of reported associations between welding exposure and poor semen quality, or that the effect of welding is non-reversible within the rather short non-exposure period.