In one boy aged 7 years with a micropenis, and in 3 boys aged 6, 4 and 5 years with delayed speech and language development and behavioural problems, Klinefelter's syndrome was diagnosed. In the first case the diagnosis was immediately suspected because of the micropenis. The other three boys were diagnosed after long observation. One of them was suspected of having a chromosomal abnormality because of behavioural problems together with a typical, female fat distribution pattern. If a delayed speech and language development and behaviour problems are present, Klinefelter's syndrome should be considered, especially if tallness and (or) small genitals are present.