The energy balance and nutritional adequacy was assessed in a group of runners with irregular menstrual function (IR, n = 10) and in a comparable group of runners with normal menstrual function (R, n = 10). Based on computerized records of 3 days food intake, the mean content of energy and macronutrients were estimated. Excess energy expenditure (EEE) during training hours was estimated using HF monitoring and individual HF/VO2 nomograms. For the rest of a daily 24 h cycle, estimated values for basal metabolic rate (BMR) and sedentary metabolic rate were applied. Daily training-related excess energy expenditure (2.1 +/- 0.3 vs 2.2 +/- 0.4) MJ (Mean +/- SEM) and total energy expenditure (TEE) (11.0 +/- 0.3 vs 11.2 +/- 0.4) MJ in R vs IR were not different in the two groups. Calculated daily energy intake (EI) was, however, significantly lower in IR (9.7 +/- 0.5 MJ) than in R (12.3 +/- 0.7 MJ), (P = 0.007). Calculated EI and TEE were in balance in R athletes. When the same calculations were applied on IR athletes, a moderate but statistically significant negative energy balance was found (-1.5 +/- 0.6 MJ, P = 0.03). The calculated energy deficit was supported by significantly lower levels of free thyroxine in IR athletes, and may indicate an adaptive lower BMR in IR athletes. Mean intakes of carbohydrates (7.0 g/kg in R and 5.7 g/kg in IR) and protein (1.71 g/kg in R and 1.49 g/kg in IR) were close to guidelines. Intakes were not significantly different in the two groups. The most important difference in macronutrient intake was found in dietary fat (1.70 g/kg in R and 1.04 g/kg in IR) that was significantly lower in the IR group than in the R group (P = 0.007). The weight stable IR and R athletes seemed to meet the recommended intake of carbohydrates, protein and fat in their diet fairly well. However, according to the present calculations, a small energy deficit was observed in IR athletes but not in R athletes. Compared with the intake of the R athletes, this energy deficit seems primarily to be related to a lower intake of dietary fat in IR athletes.