Despite their common use parental diaries of infants' cry and fuss behaviour have not been compared with objective methods of recording. To understand what is meant by the descriptions of crying and fussing in the diaries, the diaries of 10 mothers of 6 week old infants were compared with tape recordings of vocalisations made by the babies over a 24 hour period. There were moderately strong correlations between the frequency of episodes (clusters of 'negative vocalisations') on the audiotape and episodes of 'crying and fussing' in the diaries, and between the duration of episodes on the audiotape and episodes of 'crying' in the diaries. To assess the acceptability of the diaries for recording information for clinical and epidemiological research, they were then used in a population study of a wide socioeconomic group. Usable data were obtained from 91% of the sample. The results suggest that despite pronounced differences between recording methods, these diaries may provide valid and useful reports of crying and fussing in the short term.