To study the influence of season on Plasmodium vivax gametocyte carriage, the relationship between monthly rainfall and the proportion of P. vivax patients with detectable gametocytaemia was analysed. Most of the data used came from 6807 aggregated observations collected, in a refugee camp on the Thai-Burmese border, between January 2000 and December 2002. There was a positive correlation between rainfall and the incidence of P. vivax infection (Spearman's rho=+0.42; P =0.01) but the prevalence of gametocyte carriage among those with P. vivax infection was negatively correlated with rainfall (Spearman's rho=-0.58; P <0.001). The latter, negative correlation remained significant after controlling for the proportion of visitors relative to camp residents (P =0.003). Migrations, changes in transmission patterns, seasonal haematological changes, and ultraviolet immunosuppression are discussed as potential explanations for these observations.