Robinia pseudocacia stem annual growth rings from 1958 to 1987 were analyzed for 14C content. The radioactivity of annual rings was found to be strongly correlated with 14C concentration in the atmosphere, which showed a very sharp rise until 1963 due to nuclear weapon tests followed by a regular decrease from then on. Taking into account seasonal variations in both atmospheric 14C concentration and CO2 assimilation slightly improved the relation between 14C in wood and the atmosphere. A predictive curve that gives the year of wood formation as a function of radioactivity with a 5% confidence interval is shown. These results can be used to analyze wood for which traditional analysis is impossible, for example, trees with false rings or missing rings, or wood from stumps or roots.© National Research Council of Canada, NRC Research Press. Abstract reproduced by permission.