Over a period of 9 years in general practice temporary enlargement of the spleen was found in 29 episodes of pharyngitis or tonsillitis, in 2 episodes of acute upper respiratory tract infection other than pharyngitis and in 6 episodes of acute cervical lymphadenitis. In five patients more than one episode of illness associated with splenomegaly was recorded. In 26 of the 37 episodes a possible aetiology was identified. Evidence only of infection with group A streptococci was found in 14 episodes, adenoviruses or coxsackie B viruses were isolated alone in 4 episodes and in 4 episodes the only finding was the presence in the blood of more than occasional atypical mononuclear cells; in 4 episodes there was evidence of both streptococcal and viral infection. Episodes with evidence of streptococcal infection only tended to be of shorter duration and to be more evenly distributed over the year than were episodes without such evidence. Temporary splenomegaly was noted also in two children with varicella (one of whom also had streptococcal infection) and in an adult with probable rubella.