An increase in the mortality due to motor neuron disease (MND) has been reported in Spain over the past 30-40 years. It has been suggested that this increase is due to the cohort effect, but his hypothesis has not been proven. The motor neuron disease (MND) mortality statistics by age and by gender were furnished from the Spanish National Institute of Statistics. The mortality specifically by age and the adjusted age-related rates were calculated. By Poisson regression, the cohort effect of birth on the mortality by ages has been analyzed, the cohort effect also having been analyzed by graphic methods. The mortality adjusted by age was declining up until 1969, as of which time it has been on the rise. Each five-year birth cohort increases the risk of dying from MND by 8.5%. The increase in the mortality due to MND in Spain is the result of a cohort effect.