The objective of this study was to identify the key determinants of condom use with regular and casual partners among youth in Madagascar. Data stem from a reproductive health survey conducted in October–December 2000 among a representative sample of 2440 youth aged 15–24 living in Toamasina province. Following theoretical models of behaviour change, logistic regression was used to assess the effect of AIDS awareness, personal risk perception, condom access, perceived condom effectiveness, self-efficacy and social support on condom use. Among sexually experienced youth, only about four in ten males and two in ten females have ever used condoms. Fewer than 15% of youth used a condom in last intercourse with their regular partner. Whether youth will try condoms appears to depend largely on the perceived effectiveness of condoms for family planning, access to a nearby condom source, parental support for condom use, and patterns of risky sexual behaviour. Young males’ likelihood of using a condom with a regular partner increases significantly if they perceive condoms to be effective for family planning (OR=11·4; p=0·019). For females, it increases with level of self-efficacy (OR=2·1; p=0·042) and having discussed HIV prevention with someone in the last year (OR=2·8; p=0·022). Among males, condom use with casual partners is significantly higher among those who perceive themselves to be at high risk of sexually transmitted infections (OR=2·3; p=0·014), who believe condoms are effective for family planning (OR=2·8; p=0·048), who have good access to condoms (OR=2·9; p=0·002) and who perceive their parents support condom use (OR=1·7; p=0·048). In conclusion, very few youth in Toamasina are using condoms, highlighting the need to continue and expand adolescent reproductive health interventions. In this low HIV prevalence setting, it is important for these programmes to emphasize that condoms are effective for both pregnancy prevention and STI/HIV prevention.