(1) The hypothesis that individuals adjust their clutch size according to their ability to rear young has been used to explain the considerable clutch variation observed within avian populations. The hypothesis suggests that the most able individuals do best (in terms of Darwinian fitness) by having large clutches, but the less able do best by having smaller clutches. (2) To test the significance of clutch size (and original brood size) in relation to rearing ability, I manipulated 216 pairs of blue tits (P. caeruleus) breeding in Wytham Wood, England, over a 3-year period, by randomly assigning them broods of 3, 6, 9, 12 and 15. Thus, individuals laying small clutches were as likely to rear large broods as to rear small broods; the same was true for those laying large clutches. (3) In comparison with pairs laying larger clutches, adults laying smaller clutches did not feed their broods more or less often, and did not appear to suffer greater weight loss during the nestling period. The broods of parents laying smaller clutches were not significantly lighter in weight than broods reared by parents laying larger clutches. However, offspring reared by parents whose natural broods were smaller than average (2-9) survived less well than those reared by parents whose natural broads were larger than average (11-15). There was a trend for offspring survival to increase with the size of the clutch laid. (4) The brood size maximizing offspring survival did not differ with respect to a pair''s original brood size or clutch size: for all pairs, the most productive brood size was 12-15. (5) I conclude that blue tits are not individually optimizing their clutch size/brood size according to their ability to rear nestlings. Clutch size differences may reflect differences in the ability to lay and/or incubate the clutch.