Studies on sexual behaviour in Nigeria have rather been lopsided largely focussing on adolescents while neglecting the older men. This may not be unconnected with the fact that the young people are often seen as being more sexually active than the older men. In this study, the patterns and the determinants of sexual behaviour of middle-aged men were investigated. This study is based on secondary data analysis of the 2003 Nigeria Demographic and Health Survey male dataset. Data collected from a sample of 633 men whose ages fall between 40 and 59 years in Nigeria were extracted and analysed to achieve the set objectives. The variables of interest were analysed by using relevant statistical techniques with the aid of SAS enterprise guide. Sexual behaviour was measured by three variables namely: current sexual activity, extra marital partnership and condom use. Also, three hypotheses were tested. The Health Belief Model (HBM) was the theoretical model used for this study. The study shows that a high proportion of men (71.2%) aged 40-59 years in Nigeria are sexually active. The study further reveals that about 12% of Nigerian middle-aged men engage in extramarital sex and 30% are in polygynous relationships (i.e. have multiple sex partners) while condom use is very low among them. Extramarital sex is more prevalent in the rural (7.05%) than urban (4.5%) areas. The study shows that the correlates of current sexual activity among the middle-aged men in Nigeria are education and religion while engagement in extramarital sexual activity is determined by ethnicity, age at first intercourse and knowledge of HIV/AIDS. Among the sexually active ones, condom use is influenced by ethnicity, marriage type and extramarital partnership. The sexual behaviour of middle-aged men in Nigeria follows the pattern described in the HBM. That is, individual, socio-economic and HIV/AIDS factors can influence the sexual behaviour of Middle-aged men. The study concludes that in addressing the problems associated with sexual and reproductive health of Nigerian, focus should also be extended to middle-aged men, instead of the narrow focus on only adolescents and youths. Further investigation, using multiple methods of data collection is also suggested.