We examine the presence of the month effect in the foreign exchange market and its possible link with the well-documented similar seasonal anomaly in stock markets. Our main focus is on the key world currency pair, the US dollar-Deutsche mark (euro) from 1971 to 1998 (1999 to 2017). We consider it likely that seasonality in the German-US stock market returns differential is associated with seasonal capital flows to the stock market with higher returns and a similar seasonality in the foreign exchange market. Using a Markov-switching framework to account for nonlinear seasonal patterns, we detect recurrent January and December effects with similar timing in the US dollar-Deutsche mark (euro) returns, the German-US stock returns differential and the US-German net equity flows. A seasonal equity carry trade opportunity exists, as evidenced by the sign and significance of seasonal exchange rate returns, stocks returns differential and equity flows as well as their reversals.