BACKGROUND The aim of this study was to evaluate whether characteristics of human ovulation correlate with age and pregnancy potential. METHODS Two groups of women with regular menstrual cycles were included (i.e. one fertile and one infertile group), which were divided into four age groups (< or =29, 30-34, 35-39, > or = 40 years). Monitoring included observations of follicular phase length, whether ovulations occurred from the left or right ovary, the pattern of ovulations in succeeding natural cycles and, in a subset of women, early follicular phase FSH concentrations. RESULTS Ovulation moving from one ovary to the other in two consecutive cycles (i.e. contralateral ovulation) was inversely correlated with age, showing a ratio of contralateral ovulation per contra plus ipsilateral ovulations (C/C+I) of 62% in women <29 years, gradually decreasing to 42% in women >40 years. The ratio of right-sided ovulation per right plus left-sided ovulations (R/R+L) was unrelated to age and remained almost constant at a level of approximately 55%. The follicular phase length was inversely correlated with age, being 16.2 +/- 2.9, 15.4 +/- 2.9, 14.8 +/- 2.8 and 13.7 +/- 1.3 days in women < 29, 30-34, 35-39 and >40 years of age respectively. The follicular phase length was similar when comparing ovulations occurring from the right and left ovary, but comparing two successive cycles, the length of the follicular phase of the second cycle, showing contralateral ovulation, was shorter than ipsilateral ovulation with two consecutive ovulations in the same ovary. The pregnancy rate of the four groups decreased with age, being 14, 12, 5 and 3% respectively. The C/C+I ratio correlates with pregnancy rate and follicular phase length, and inversely correlates with basal FSH, whereas R/R+L is unrelated to age and pregnancy rate. CONCLUSIONS Human ovulation shows characteristics related to age. The interaction between the two ovaries seems to be most pronounced in the younger years, where ovulations jump from one ovary to the other more frequently than later on in life. The C/C+I ratio shows a clear correlation with age and pregnancy rate.