The relationship between serum total testosterone (T) level and fluid intelligence (Cattell's Culture Fair Intelligence Test) was studied in young men and women. There was no significant difference between IQs of men and women. IQ tended to increase with T in men, except at very high T levels. It was concluded that (i) T may be associated with IQ, even in samples with no sex-related IQ difference; (ii) too low or too high T may be disadvantageous for fluid intelligence in women; (iii) T may be advantageous for this kind of IQ in men, except very high T levels.