We monitored mortality and recruitment annually for six years for all trees ≥5 cm diameter at breast height in four 1-ha plots in second-growth, lowland rain forests in northeastern Costa Rica. Stands initially ranged in age from 12 to 25 years since abandonment of cattle pastures. In younger stands, abundance decreased 10–20% in the small size class (5–9.9 cm dbh), but increased 49–100% in the large size class (≥25 cm dbh). In the two youngest plots, 45.0% and 27.6% of the common tree species showed annual population increases of 3% or more, whereas 10% showed annual decreases of 4% or more. Annual rates of tree mortality for all size classes and recruitment into the 5-cm size class varied significantly among years, and mortality rates of trees 5–9.9 cm dbh were higher in younger than in older stands. Tree mortality was highest during the 1997–1998 El Niño Southern Oscillation (ENSO) year; on average, 5.6%, 3.5%, and 1.4% of trees died in the small, intermediate, and large size classes, respectively. Mortality rates during the ENSO year were 1.9 times higher for trees ≥10 cm dbh compared to non-ENSO years. Annual mortality rates for trees ≥10 cm dbh decreased significantly with increasing dry-season rainfall, but not with total annual rainfall during the census periods. Tree mortality (particularly for trees ≥25 cm dbh) in these secondary forests was highly sensitive to dry-season rainfall, even during non-ENSO years. Increased tree mortality during ENSO years is likely to accelerate forest dynamics and successional change in wet tropical second-growth forests.