Background and aims. Fatty liver may have different determinants in normal-weight and in obese individuals. We measured factors associated with fatty liver in 863 normal-weight (BMI < 25) and 1135 overweight/obese (BMI ≥ 25) young and middle-aged adults (45% male, age 34-49 years) in the population-based Cardiovascular Risk in Young Finns Study. Methods and results. The prevalence of fatty liver detected with ultrasound was 29% in overweight/obese and 5% in normal-weight participants. In overweight/obese, the independent correlates were waist circumference (odds ratio for 1 standard deviation increase = 3.78), alanine transaminase (2.11), BMI (2.00), male sex (1.74), triglycerides (1.44), systolic blood pressure (1.31), fasting insulin (1.23), and physical activity (0.76). In normal weight, the independent correlates included alanine transaminase (3.05), smoking (2.56), systolic blood pressure (1.54), and alcohol intake (1.41). In normal-weight participants, the associations with fatty liver were stronger for alcohol intake and smoking, and weaker for triglycerides, than in overweight/obese participants (P for interaction < 0.05). Conclusion. Prevalence of fatty liver was 29% in overweight/obese and 5% in normal-weight adults. Differences in factors associated with fatty liver were seen between these two groups: alcohol intake and smoking were more strongly and triglycerides more weakly associated in normal-weight than in overweight/obese participants.