Fifty-one mother-infant pairs were followed prospectively during the period 3 days to 18 months after delivery. In total 54.9% of the mothers experienced transient lactation crises, emanating mostly from a perception of breast milk insufficiency. Within the crisis group no significant difference in the infants' intake of breast milk during the crises compared with control measurements 1 week later was found. Nor had the crises any immediate impact on growth of the infants. A comparison between the crisis and the non-crisis group, revealed that the breast milk consumption in the crisis group was throughout lower with significant differences at 3 and 5 months. The infants in the crisis group also had a significantly lower weight at 2, 3, 4 and 9 months and were significantly thinner for their height at 1-6 months and at 9 months, although both groups were above the NCHS mean. We conclude that even if the infants in the crisis group had a lower consumption and a slower growth development, the differences were comparatively small. Furthermore, evidence was provided that the breast milk insufficiency occasionally perceived as acute by the mothers was in most cases not real.